rpc: avoid possible recursion in create_connection
    
    There was a recursion in RPCClient.create_connection() which under rare
    circumstances would not have an ending condition. This commit removes
    it and cleans up the code a bit as well.
    
    https://pagure.io/freeipa/issue/6796
    
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>