Updated and refactored images fixtures

*Refactored generate_images_client method
*Made serilize_format, deserialize_format, and url a part of the class
*Updateed setUpClass to call the new generate_images_client method when creating clients
*Replace "override_url" with "base_url" as it is passed in
*Removed new lines for consistency
*Made summary documentation consistent
*Added authentication failure check for admin_access_data
*Added public endpoint failure check
*Refactored endpoint validation check
*Added comment to explain the endpoint check

Change-Id: I69b6256d57bb5f7e29372260ab8f42586a2c8d15
Merge "Updated and refactored images fixtures"