Warning the user when using a loopback IP as forwarder
    
    Changing the --forwarder option to accept a loopback IP.
    Previously, an error would be raised, now we just show a
    warning message.
    
    Fixes: https://pagure.io/freeipa/issue/5801
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>