This patch adds coverage for the various exceptions handled and
raised in get, get_size and delete methods.
It also corrects the behavior of _test_cinder_get_size method where
client.volumes returned dictionary instead of a MagicMock and it
worked due to the existing method "get" in dictionary which has
same name as "get" method in cinderclient. The dictionary object
is replaced with the appropriate MagicMock object in this patch.
Change-Id: If63a6c810b5aab992e54857bc81f5052c2c593c4
4bd0304efb