Object-ify APIv3 availability_zone extension

This makes the availablility_zone extension use the Service object
for looking up service details instead of a direct-to-database
call.

Related to blueprint compute-manager-objects-juno

Change-Id: I3091bc06a74e32871a743a831ab3db23220ae8f8
Merge "Object-ify APIv3 availability_zone extension"