always ask the resolver for the reverse zone when manipulating PTR records
    
    Instead of searching for all zones to identify the correct reverse zone, we
    will first ask the resolver to return the name of zone that should contain the
    desired record and then see if IPA manages this zone.
    
    This patch also removes a duplicate function in bindinstance.py that is not
    used anywhere.
    
    https://fedorahosted.org/freeipa/ticket/5200
    
    Reviewed-By: Petr Spacek <pspacek@redhat.com>