Image API v2.0 Unshare Previously Shared Image

Implements unshare previously shared image test:
 - added a new test scenario - unshare previously shared
 image between two users.
 - reverted changes on the image fixtures and made it a separate PR.
 - improved naming of variables.

Refactors test to add tags.

Change-Id: I9b9a4e3f29a728d341c32ac0b1fe1a4cfa957b3b
Merge "Image API v2.0 Unshare Previously Shared Image"