Refactors nova.image.glance unit tests for show()

This patches continues a patch series to cleanup and streamline the unit
tests for the nova.image.glance module. Here, we create true unit tests
for the GlanceImageService.show() method, stressing all code paths and
edges but only for that specific method, using mocks and no fake glance
image service.

This patch removes "unit" tests from the TestGlanceImageService that
called glance.GlanceImageService.show(), as those tests used the fake
glance service and the code paths are now more than covered by these new
mock-using tests.

Change-Id: I200a9e9812d6de011fa6faa8fb8dc570075ce902
Related-bug: #1293938
Merge "Refactors nova.image.glance unit tests for show()"