[supervisor] Install vanilla package once again, install Python 3 version in Buster container (#5546)

**- Why I did it**

We were building a custom version of Supervisor because I had added patches to prevent hangs and crashes if the system clock ever rolled backward. Those changes were merged into the upstream Supervisor repo as of version 3.4.0 (http://supervisord.org/changes.html#id9), therefore, we should be able to simply install the vanilla package via pip. This will also allow us to easily move to Python 3, as Python 3 support was added in version 4.0.0.

**- How I did it**

- Remove Makefiles and patches for building supervisor package from source
- Install Python 3 supervisor package version 4.2.1 in Buster base container
    - Also install Python 3 version of supervisord-dependent-startup in Buster base container
- Debian package installed binary in `/usr/bin/`, but pip package installs in `/usr/local/bin/`, so rather than update all absolute paths, I changed all references to simply call `supervisord` and let the system PATH find the executable to prevent future need for changes just in case we ever need to switch back to build a Debian package, then we won't need to modify these again.
- Install Python 2 supervisor package >= 3.4.0 in Stretch and Jessie base containers

dockers/docker-database/docker-database-init.sh

@@ -55,7 +55,7 @@ if [[ $DATABASE_TYPE == "chassisdb" ]]; then
     # generate all redis server supervisord configuration file
     sonic-cfggen -j $db_cfg_file_tmp -t /usr/share/sonic/templates/supervisord.conf.j2 > /etc/supervisor/conf.d/supervisord.conf
     rm $db_cfg_file_tmp
-    exec /usr/bin/supervisord
+    exec supervisord
     exit 0
 fi
 
@@ -79,4 +79,4 @@ else
 fi
 rm $db_cfg_file_tmp
 
-exec /usr/bin/supervisord
+exec supervisord