Add unit test coverage for backup chunkeddriver class
Add unit test coverage for backup chunkeddriver class

Only indirect test coverage exists for this abstract base
class. This patch expands testing coverage specifically
for this class's methods.

Change-Id: I5cdb8677f7a137f35a8f4b24ea4a4b0a40e76e9c

phenoxim/cinder,openstack/cinder,j-griffith/cinder,j-griffith/cinder,phenoxim/cinder,mahak/cinder,mahak/cinder,openstack/cinder