Update gate_hook to support multiple drivers
Update gate_hook to support multiple drivers

The intent here is to begin allowing us to gate on more than
just PowerDNS.

Change-Id: Ib91a50a18293da672aedd590fc412a97fae3d348

kiall/designate-py3,grahamhayes/designate,muraliselva10/designate,ramsateesh/designate,kiall/designate-py3,kiall/designate-py3,tonyli71/designate,openstack/designate,kiall/designate-py3,cneill/designate-testing,tonyli71/designate,cneill/designate,grahamhayes/designate,kiall/designate-py3,openstack/designate,cneill/designate-testing,cneill/designate,ionrock/designate,ionrock/designate,cneill/designate,ramsateesh/designate,muraliselva10/designate,ramsateesh/designate,openstack/designate,tonyli71/designate,ionrock/designate,muraliselva10/designate,grahamhayes/designate,cneill/designate-testing,cneill/designate,cneill/designate