Introduce optional service_registration stanza (#7887)

* move ServiceDiscovery into methods

* add ServiceDiscoveryFactory

* add serviceDiscovery field to vault.Core

* refactor ConsulServiceDiscovery into separate struct

* cleanup

* revert accidental change to go.mod

* cleanup

* get rid of un-needed struct tags in vault.CoreConfig

* add service_discovery parser

* add ServiceDiscovery to config

* cleanup

* cleanup

* add test for ConfigServiceDiscovery to Core

* unit testing for config service_discovery stanza

* cleanup

* get rid of un-needed redirect_addr stuff in service_discovery stanza

* improve test suite

* cleanup

* clean up test a bit

* create docs for service_discovery

* check if service_discovery is configured, but storage does not support HA

* tinker with test

* tinker with test

* tweak docs

* move ServiceDiscovery into its own package

* tweak a variable name

* fix comment

* rename service_discovery to service_registration

* tweak service_registration config

* Revert "tweak service_registration config"

This reverts commit 5509920a8ab4c5a216468f262fc07c98121dce35.

* simplify naming

* refactor into ./serviceregistration/consul

command/commands.go

@@ -62,6 +62,9 @@ import (
 	physZooKeeper "github.com/hashicorp/vault/physical/zookeeper"
 	physFile "github.com/hashicorp/vault/sdk/physical/file"
 	physInmem "github.com/hashicorp/vault/sdk/physical/inmem"
+
+	sr "github.com/hashicorp/vault/serviceregistration"
+	csr "github.com/hashicorp/vault/serviceregistration/consul"
 )
 
 const (
@@ -155,6 +158,10 @@ var (
 		"raft":                   physRaft.NewRaftBackend,
 		"zookeeper":              physZooKeeper.NewZooKeeperBackend,
 	}
+
+	serviceRegistrations = map[string]sr.Factory{
+		"consul": csr.NewConsulServiceRegistration,
+	}
 )
 
 // Commands is the mapping of all the available commands.
@@ -517,9 +524,12 @@ func initCommands(ui, serverCmdUi cli.Ui, runOpts *RunOptions) {
 				CredentialBackends: credentialBackends,
 				LogicalBackends:    logicalBackends,
 				PhysicalBackends:   physicalBackends,
-				ShutdownCh:         MakeShutdownCh(),
-				SighupCh:           MakeSighupCh(),
-				SigUSR2Ch:          MakeSigUSR2Ch(),
+
+				ServiceRegistrations: serviceRegistrations,
+
+				ShutdownCh: MakeShutdownCh(),
+				SighupCh:   MakeSighupCh(),
+				SigUSR2Ch:  MakeSigUSR2Ch(),
 			}, nil
 		},
 		"ssh": func() (cli.Command, error) {