Images API v2.0 DELETE image/tags tests

blueprint glance-api-v2-tests

Tests against the DELETE image/tags functionality/endpoint of the Glance
API V2.0.

- remove custom assertion message
- extract assertion message to fixture as a constant.
- add assertion messages.
- make not implmemented tests fail.
- remove `net` attribute in test tags.

Change-Id: Ic272b4567f6575789bbe895fd94f5451f17b0718
Merge "Images API v2.0 DELETE image/tags tests"