Images API v2.0 GET image tests (cont.)

Part 2 of Images API v2.0 GET image tests. Adding positive and negative
tests.

blueprint glance-api-v2-tests

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

- replace PRIVATE constant with class enum.

Depends on:
- https://review.openstack.org/#/c/50561/
- https://review.openstack.org/#/c/46954/

Change-Id: I4d481212767992ff9f858bfd05ed22ee2c3530ae
Merge "Images API v2.0 GET image tests (cont.)"