Decorate AvailabilityZonePluginBase properly
Decorate AvailabilityZonePluginBase properly

Without this, users could improperly overlook either
get_availability_zones or validate_availability_zones.

Change-Id: I6530dca041a92da2f1d0bf01d359e5edea3df5c0
Closes-Bug: #1573388

openstack/neutron,noironetworks/neutron,huntxu/neutron,igor-toga/local-snat,mahak/neutron,noironetworks/neutron,openstack/neutron,cloudbase/neutron,bigswitch/neutron,huntxu/neutron,mahak/neutron,bigswitch/neutron,eayunstack/neutron,igor-toga/local-snat,eayunstack/neutron,cloudbase/neutron,openstack/neutron,mahak/neutron,sebrandon1/neutron,sebrandon1/neutron