RBD unit test improvements

This commit brings the implementation of the test_rbd and
test_backup_ceph unit tests more inline and tries to ensure
that no mock is set without being unset so as not to tread
on the toes of any other tests.

Change-Id: Ibb6bd3bf90952914512e3fa39fd216f43f0f8ecb
Closes-bug: 1276657
Merge "RBD unit test improvements"