Images API v2.0 Image Discovery Of Available Images To User

Implements image discovery of available images to user test:
 - added new test scenario 'image discovery of available images to user'.
 - refactored image fixtures.
 - reverted changes on the image fixtures and made it a separate PR.
 - improved naming of variables.

Refactors the test:
  - added type and regression tags.
 - fixed variable naming
 - fixed import order
 - removed admin user
 - improved the test name in changing 'discovery' to visibility
 - added a new line.
 - change file name and method name from
   test_image_visibility_of_images_available_to_user.py
   to test_visibility_of_images_shared_between_users.py
 - change class name from ImageVisibilityOfImagesAvailableToUserTest
   to VisibilityOfImagesSharedBetweenUsersTest

Change-Id: Iebb9b8c4a57b8ee4f8fc52e7532272b26ce24dea
Merge "Images API v2.0 Image Discovery Of Available Images To User"