Images API v2.0 PATCH image tests (cont...)

Implements new test scenarios for 'test_patch_image.py':
 - refactored test_patch_image.py
 - added more test cases for image update.
 - made some small refactoring to improve readability.

Change-Id: I2506a53849ebe294954e653b3f943a44952c34b2
Merge "Images API v2.0 PATCH image tests (cont...)"