CIP-1933 Fix group proxy FQDN add

Summary:

Fix bug where fqdns are added improperly, leading to vccs not being removed from the group vcc.

Description:

• Noticed unexpected behaviour where VCCs were not being removed from group_vcc
  • this led to errors when the system tried to configure the group_vcc and included receptors that we weren't expecting
• we tracked it down to the group_proxy.add(), which was using extend to add a list of fqdns; however, the MCS has been passing in a single fqdn at a time as a string, so that string was interpreted as a list: e.g., 'fqdn_1' would be interpreted as as: `['f', 'q', 'd', 'n', '_', '1']`.
• Modified group_proxy.add() to append fqdns instead of extending
• Modified group_proxy.add() and group_proxy.remove() to expect a str fqdn instead of List[str] of fqdns
• Updated integration tests accordingly, now that the group_proxy.add()/remove() is implemented correctly