Image API v2 Image Membership Sharing Cycle

Implements image membership sharing cycle:
 - added a new test scenario
 - refactored image fixtures.
 - reverted changes on the image fixtures and made it a separate PR.
 - improve naming of variables.

Refactors the test
 - added tags
 - reorder the imports
 - removed duplicate lines
 - removed any reference to admin user or tenant
 - added new test steps
 - made changes to get the test run against preprod
 - rebased fixtures to the current changes on the master.

Rectify step 10 to refer to "third tenant" instead of "alternative tenant".

Change-Id: Icd9a33ebb916474c0f1cf9a9ccb021676da1268d
Merge "Image API v2 Image Membership Sharing Cycle"