Images API v2.0 DELETE image tests

blueprint glance-api-v2-tests

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

- add assertion message.
- make not implmemented tests fail.
- remove `net` attribute in test tags.
- remode new line in the imports.
- remove custom assertion message throughout these tests.

Change-Id: Ic3cee1042d65b3a769a6cd2d9ddd17601f9291fb
Merge "Images API v2.0 DELETE image tests"