Refactors unit tests of image service detail()

We do not need to have "unit" tests that test the things like
pagination work with the glanceclient.images.detail() call. glanceclient
and glance have unit tests for that. What the unit tests for the glance
image service detail() method in Nova need is to test only the code
boundary that exists within the detail() method -- i.e. verify that the
calls to the glanceclient and other helper methods are made with the
right parameters.

This patch pulls the various unit tests out of the
TestGlanceImageService test class and into their own test class that
uses mocks and verifies each code path within the detail() method.

Next up is the create, update and delete methods of the
GlanceImageService class. The final piece is to rework the download unit
tests (which aren't unit tests but more functional tests that take
around a second each to run :(

Change-Id: I94897ff54a36a1af4258ba4a3c88b45c079f4bb5
Related-bug: #1293938
Merge "Refactors unit tests of image service detail()"