DNS: allow to add forward zone to already broken sub-domain
    
    Errors during DNS resolution might indicate that forwarder is the
    necessary configuration which is missing. Now we disallow adding a
    forwarder only if the zone is normally resolvable without the forwarder.
    
    https://fedorahosted.org/freeipa/ticket/6062
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>