Convert glance unit tests to not use stubs

Takes unit tests that were testing some helper functions in the
nova.image.glance module out of the TestGlanceImageService test case
class and puts them in separate test classes that do not use the
nova.tests.glance.stubs module -- instead, I converted them to just use
the simpler mock library and only test the specific things that the unit
tests were supposed to test.

Change-Id: I2c19268ce6a20eaaed99eda97f63cd1babd5e380
Merge "Convert glance unit tests to not use stubs"