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
diff --git a/releasenotes/notes/add-volume-quota-class-client-as-library-c4c2b22c36ff807e.yaml b/releasenotes/notes/add-volume-quota-class-client-as-library-c4c2b22c36ff807e.yaml
new file mode 100644
index 0000000..e6847eb
--- /dev/null
+++ b/releasenotes/notes/add-volume-quota-class-client-as-library-c4c2b22c36ff807e.yaml
@@ -0,0 +1,8 @@
+---
+features:
+  - |
+    Define v2 quota_classes_client for the volume service as library
+    interfaces, allowing other projects to use this module as stable libraries
+    without maintenance changes.
+
+    * quota_classes_client(v2)