Code dedup in glance/tests/unit/v1/test_registry_api.py

Move duplicated fixture set-ups, requests and assertions to tests.utils.py

bp glance-tests-code-duplication

Change-Id: Ie0f13b5f5d5a242f24ac7cd6cc548ff09ebe6ba5
Merge "Code dedup in glance/tests/unit/v1/test_registry_api.py"