This patch aims at a refactoring effort that would remove
duplicate tests (current and future) by moving them into a
common base class which is called via both single and multi store
test modules with their specific configurations.
This has a lot of benefits:
1) Removes duplicate code
2) Makes addition of new tests easier and cleaner
3) Ensuring a new method/code path added is tested in both
single and multi store configurations
4) Fixing issues detected while refactoring methods
(Eg: tests for add method in test_multistore_cinder were not
passing the hashing_algo parameter which is currently handled
by the backward compat code (back_compat_add decorator) but
those tests will break when we remove backward compatibility)
Change-Id: I12569af5623f1cd7803c00a6c3b9eb211f15b6fd
fba6d0dd83