Change v3 availability-zone API to v2.1

This patch changes v3 availability-zone API to v2.1 and makes v2
unit tests share between v2 and v2.1.

The differences between v2 and v3 are described on the wiki page
https://wiki.openstack.org/wiki/NovaAPIv2tov3.

Partially implements blueprint v2-on-v3-api

Change-Id: Ifbd152e431d182a5ccabac7fcc84fcc8a2fc6e5a
Merge "Change v3 availability-zone API to v2.1"