Images API v2.0 GET image tests

blueprint glance-api-v2-tests

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

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

Change-Id: I965e7b6e77903b61a30221ca4f736b8bb10fe437