[sonic-cfggen]: Use unix socket when reading from DB only if we are using sudo. (#7002)

Closes issue #6982.
The issue was root caused as we were using the unix_socket for reading from DB as a default mechanism (#5250). The redis unix socket is created as follows.

admin@str--acs-1:~$ ls -lrt /var/run/redis/redis.sock 
srwxrw---- 1 root redis 0 Mar  6 01:57 /var/run/redis/redis.sock
So it used to work fine for the user "root" or if user is part of redis group ( admin was made part of redis group by default )

Check if the user is with sudo permissions then use the redis unix socket, else fallback to tcp socket.

Author: judyjoseph

src/sonic-config-engine/sonic-cfggen

@@ -350,10 +350,11 @@ def main():
         deep_update(data, json.loads(args.additional_data))
 
     if args.from_db:
+        use_unix_sock = True if os.getuid() == 0 else False
         if args.namespace is None:
-            configdb = ConfigDBPipeConnector(use_unix_socket_path=True, **db_kwargs)
+            configdb = ConfigDBPipeConnector(use_unix_socket_path=use_unix_sock, **db_kwargs)
         else:
-            configdb = ConfigDBPipeConnector(use_unix_socket_path=True, namespace=args.namespace, **db_kwargs)
+            configdb = ConfigDBPipeConnector(use_unix_socket_path=use_unix_sock, namespace=args.namespace, **db_kwargs)
 
         configdb.connect()
         deep_update(data, FormatConverter.db_to_output(configdb.get_config()))