Images API v2.0 GET images tests

blueprint glance-api-v2-tests

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

- style issues and removed test_get_images since it's redundant
- make name changes consistent
- remove custom assertion messages
- move variables to config file
- address naval's comments
- address luke's comments
- make not implmemented tests fail.
- remove `net` attribute in test tags.

Change-Id: Ib40a97dc37ffe7bed839577d5845378145c258c4
Merge "Images API v2.0 GET images tests"