dns: do not add (forward)zone if it is already resolvable.
    
    Check if the zone user wants to add is already resolvable and refuse to
    create it if yes. --skip-overlap-check and --force options suppress this check.
    
    https://fedorahosted.org/freeipa/ticket/5087
    
    Reviewed-By: Petr Spacek <pspacek@redhat.com>