Enable some volumes v2 tests by sharing codes part2

1. Rename volumes v1 client to a base client, then create
   v1 and v2 clients inheriting base client.
2. create volumes v2 tests using the existent test class.
3. create a new test class for v1, which inherits v2.

This patch includes the following volumes tests:
- test_availability_zone.py
- test_extensions.py

Partially implements: blueprint cinder-v2-api-tests

Change-Id: Ie21ebc184710e9abe894cdc20ed7a39aacc94ded
Merge "Enable some volumes v2 tests by sharing codes part2"