[dhcp-relay] make DHCP relay an extension (#6531)

- Why I did it
Make DHCP relay docker an extension. DHCP relay now carries dhcp relay commands CLI plugin and has a complete manifest.
It is installed as extension if INCLUDE_DHCP_REALY is set to y.

DEPENDS on #5939

- How I did it
Modify DHCP relay docker makefile and dockerfile. Make changes to sonic_debian_extension.j2 to install sonic packages.
I moved DHCP related CLI tests from sonic-utilities to DHCP relay docker.
This PR introduces a way to write a plugin as part of docker image and run the tests from cli-plugin-tests directory under docker directory.
The test result is available in target/docker-dhcp-relay.gz.log:

[ REASON ] :      target/docker-dhcp-relay.gz does not exist   NON-EXISTENT PREREQUISITES: docker-start target/docker-config-engine-buster.gz-load target/python-wheels/sonic_utilities-1.2-py3-none-any.whl-in
stall target/debs/buster/python3-swsscommon_1.0.0_amd64.deb-install
[ FLAGS  FILE    ] : []
[ FLAGS  DEPENDS ] : []
[ FLAGS  DIFF    ] : []
============================= test session starts ==============================
platform linux -- Python 3.7.3, pytest-3.10.1, py-1.7.0, pluggy-0.8.0 -- /usr/bin/python3
cachedir: .pytest_cache
rootdir: /sonic/dockers/docker-dhcp-relay/cli-plugin-tests, inifile:
plugins: cov-2.6.0
collecting ... collected 10 items

test_config_dhcp_relay.py::TestConfigVlanDhcpRelay::test_plugin_registration PASSED [ 10%]
test_config_dhcp_relay.py::TestConfigVlanDhcpRelay::test_config_vlan_add_dhcp_relay_with_nonexist_vlanid PASSED [ 20%]
test_config_dhcp_relay.py::TestConfigVlanDhcpRelay::test_config_vlan_add_dhcp_relay_with_invalid_vlanid PASSED [ 30%]
test_config_dhcp_relay.py::TestConfigVlanDhcpRelay::test_config_vlan_add_dhcp_relay_with_invalid_ip PASSED [ 40%]
test_config_dhcp_relay.py::TestConfigVlanDhcpRelay::test_config_vlan_add_dhcp_relay_with_exist_ip PASSED [ 50%]
test_config_dhcp_relay.py::TestConfigVlanDhcpRelay::test_config_vlan_add_del_dhcp_relay_dest PASSED [ 60%]
test_config_dhcp_relay.py::TestConfigVlanDhcpRelay::test_config_vlan_remove_nonexist_dhcp_relay_dest PASSED [ 70%]
test_config_dhcp_relay.py::TestConfigVlanDhcpRelay::test_config_vlan_remove_dhcp_relay_dest_with_nonexist_vlanid PASSED [ 80%]
test_show_dhcp_relay.py::TestVlanDhcpRelay::test_plugin_registration PASSED [ 90%]
test_show_dhcp_relay.py::TestVlanDhcpRelay::test_dhcp_relay_column_output PASSED [100%]

=============================== warnings summary ===============================
/usr/local/lib/python3.7/dist-packages/tabulate.py:7
  /usr/local/lib/python3.7/dist-packages/tabulate.py:7: DeprecationWarning: Using or importing the ABCs from 'collections' instead of from 'collections.abc' is deprecated, and in 3.8 it will stop working
    from collections import namedtuple, Iterable

-- Docs: https://docs.pytest.org/en/latest/warnings.html
==================== 10 passed, 1 warnings in 0.35 seconds =====================

dockers/docker-dhcp-relay/cli-plugin-tests/mock_tables.py

@@ -0,0 +1,154 @@
+# MONKEY PATCH!!!
+import json
+import os
+from unittest import mock
+
+import mockredis
+import redis
+import swsssdk
+from sonic_py_common import multi_asic
+from swsssdk import SonicDBConfig, SonicV2Connector, ConfigDBConnector, ConfigDBPipeConnector
+from swsscommon import swsscommon
+
+
+topo = None
+dedicated_dbs = {}
+
+def clean_up_config():
+    # Set SonicDBConfig variables to initial state
+    # so that it can be loaded with single or multiple
+    # namespaces before the test begins.
+    SonicDBConfig._sonic_db_config = {}
+    SonicDBConfig._sonic_db_global_config_init = False
+    SonicDBConfig._sonic_db_config_init = False
+
+def load_namespace_config():
+    # To support multi asic testing
+    # SonicDBConfig load_sonic_global_db_config
+    # is invoked to load multiple namespaces
+    clean_up_config()
+    SonicDBConfig.load_sonic_global_db_config(
+        global_db_file_path=os.path.join(
+            os.path.dirname(os.path.abspath(__file__)), 'database_global.json'))
+
+def load_database_config():
+    # Load local database_config.json for single namespace test scenario
+    clean_up_config()
+    SonicDBConfig.load_sonic_db_config(
+        sonic_db_file_path=os.path.join(
+            os.path.dirname(os.path.abspath(__file__)), 'database_config.json'))
+
+
+_old_connect_SonicV2Connector = SonicV2Connector.connect
+
+def connect_SonicV2Connector(self, db_name, retry_on=True):
+    # add topo to kwargs for testing different topology
+    self.dbintf.redis_kwargs['topo'] = topo
+    # add the namespace to kwargs for testing multi asic
+    self.dbintf.redis_kwargs['namespace'] = self.namespace
+    # Mock DB filename for unit-test
+    global dedicated_dbs
+    if dedicated_dbs and dedicated_dbs.get(db_name):
+        self.dbintf.redis_kwargs['db_name'] = dedicated_dbs[db_name]
+    else:
+        self.dbintf.redis_kwargs['db_name'] = db_name
+    self.dbintf.redis_kwargs['decode_responses'] = True
+    _old_connect_SonicV2Connector(self, db_name, retry_on)
+
+def _subscribe_keyspace_notification(self, db_name, client):
+    pass
+
+
+def config_set(self, *args):
+    pass
+
+
+class MockPubSub:
+    def get_message(self):
+        return None
+
+    def psubscribe(self, *args, **kwargs):
+        pass
+
+    def __call__(self, *args, **kwargs):
+        return self
+
+    def listen(self):
+        return []
+
+    def punsubscribe(self, *args, **kwargs):
+        pass
+
+    def clear(self):
+        pass
+
+INPUT_DIR = os.path.dirname(os.path.abspath(__file__))
+
+
+class SwssSyncClient(mockredis.MockRedis):
+    def __init__(self, *args, **kwargs):
+        super(SwssSyncClient, self).__init__(strict=True, *args, **kwargs)
+        # Namespace is added in kwargs specifically for unit-test
+        # to identify the file path to load the db json files.
+        topo = kwargs.pop('topo')
+        namespace = kwargs.pop('namespace')
+        db_name = kwargs.pop('db_name')
+        self.decode_responses = kwargs.pop('decode_responses', False) == True
+        fname = db_name.lower() + ".json"
+        self.pubsub = MockPubSub()
+
+        if namespace is not None and namespace is not multi_asic.DEFAULT_NAMESPACE:
+            fname = os.path.join(INPUT_DIR, namespace, fname)
+        elif topo is not None:
+            fname = os.path.join(INPUT_DIR, topo, fname)
+        else:
+            fname = os.path.join(INPUT_DIR, fname)
+
+        if os.path.exists(fname):
+            with open(fname) as f:
+                js = json.load(f)
+                for k, v in js.items():
+                    if 'expireat' in v and 'ttl' in v and 'type' in v and 'value' in v:
+                        # database is in redis-dump format
+                        if v['type'] == 'hash':
+                            # ignore other types for now since sonic has hset keys only in the db
+                            for attr, value in v['value'].items():
+                                self.hset(k, attr, value)
+                    else:
+                        for attr, value in v.items():
+                            self.hset(k, attr, value)
+
+    # Patch mockredis/mockredis/client.py
+    # The offical implementation assume decode_responses=False
+    # Here we detect the option and decode after doing encode
+    def _encode(self, value):
+        "Return a bytestring representation of the value. Taken from redis-py connection.py"
+
+        value = super(SwssSyncClient, self)._encode(value)
+
+        if self.decode_responses:
+           return value.decode('utf-8')
+
+    # Patch mockredis/mockredis/client.py
+    # The official implementation will filter out keys with a slash '/'
+    # ref: https://github.com/locationlabs/mockredis/blob/master/mockredis/client.py
+    def keys(self, pattern='*'):
+        """Emulate keys."""
+        import fnmatch
+        import re
+
+        # Make regex out of glob styled pattern.
+        regex = fnmatch.translate(pattern)
+        regex = re.compile(regex)
+
+        # Find every key that matches the pattern
+        return [key for key in self.redis if regex.match(key)]
+
+
+swsssdk.interface.DBInterface._subscribe_keyspace_notification = _subscribe_keyspace_notification
+mockredis.MockRedis.config_set = config_set
+redis.StrictRedis = SwssSyncClient
+SonicV2Connector.connect = connect_SonicV2Connector
+swsscommon.SonicV2Connector = SonicV2Connector
+swsscommon.ConfigDBConnector = ConfigDBConnector
+swsscommon.ConfigDBPipeConnector = ConfigDBPipeConnector