Port AvailabilityZone API to v3 Part 2

This patch contains the changes required to adapt the AvailabilityZone
extension and the corresponding unittest to the v3 framework. Moves the
commented out tests from the v3 version of test_servers to the config
drive extension unittests.

Partially implements: bp v3-api-extension-versioning

Change-Id: I2cf214061e3accbb056ca53c224df1c5bf74a6e4
Merge "Port AvailabilityZone API to v3 Part 2"