Add test cases for volume quota class

Now tempest has provided tests for tenant's quota in volume, but
lacks tests for volume quota class. This patch adds this support.

Including:

[1] Add volume v2 quota class client as library
[2] Add release note
[3] Add test cases: show & update volume 'default' quota class
[4] Add unit tests for volume v2 quota class client
[5] Fix for test_volume_quotas.py

Change-Id: I30bac79b986e6e90d43dcc6f9d247e74a314bf3c
8 files changed