Images API v2.0 Image Visibility Life Cycle Test

Implements image visibility life cycle test:
 - added image a new test scenario - image visibility life cycle test.
 - refactored images fixtures to add needed test data.
 - reverted changes made on the image fixtures and made it a separate PR.
 - improved naming of variables.

Refactors the test:
 - added type and regression tags
 - added test steps and made necessary change to run the test on preprod
 - removed any reference of admin user and used third_user.
 - correcred the branch name.
 - reordered the imports.

Change-Id: Ic9e5866eb3f58080e1edd8e978feeee77696b3fd
Merge "Images API v2.0 Image Visibility Life Cycle Test"