Cinder Tempest plugin tests: Switch to stable interface

All the cinder tempest plugin tests are inherited from tempest/api/volume/base.py
which is not stable class and can be changed anytime by tempest.

One recent case where some changes in base class on tempest side could
have break the cinder tests-
 http://logs.openstack.org/07/522707/3/check/legacy-tempest-dsvm-full-devstack-plugin-ceph/a92cd60/logs/testr_results.html.gz

This commit switch cinder tests to use test.py which is stable interface
for plugin and create a base class with commonly used function & variable.

Change-Id: I0edda1fa23aed0b3d25691d5f4440a7234decc9a
Closes-Bug: #1734821
5 files changed