Images API v2.0 GET image/file data tests

blueprint glance-api-v2-tests

Tests against the GET image/file 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: I2bfb89c98d5d8f3bc5aca833c5792d6a2019a5ae
Merge "Images API v2.0 GET image/file data tests"