Mock glance client object in version unit tests

This patch changes the glance client version unit tests
to mock the Client object completely.  Previously the
tests were ensuring the right version of the client was
returned, but that required too much knowledge of glance's
implementation and ended up breaking the tests when glance
changed the implementation details of the Client class.

The new code tests if cinder is calling the integration
point correctly for the version, rather than if glance
is correctly returning the right client; that should be
a glance test, not a cinder test.

Closes-Bug: 1366596
Change-Id: I979175e6c3b5f98076bde6d36ca52fb8b03009b8