Removes GlanceClient stubs

Refactors the tests in nova.tests.image.test_glance that were checking
the behaviour of both the GlanceClientWrapper retry logic and the
glanceclient.Client creation to use mock instead of the FakeGlanceClient
in nova.tests.glance.stubs. The fake stub client was actually masking
issues in the existing test cases, including not properly checking the
identity headers that are actually supplied to the real
glanceclient.Client constructor.

Change-Id: I1c114df8e4ab2fccd966ed4af22181881590c443
Partial-bug: #1293938
Merge "Removes GlanceClient stubs"