Images API v2.0 POST tags tests (cont...)

Implements new test scenarios for 'test_post_tags.py':
  - refactored previous test for consistency.
  - added more test cases for add tags to image.

Refactors the tests:
 - improved readability.

Change-Id: Iad33629f49b196b2c9eb738c75ef5276e9382c96
Merge "Images API v2.0 POST tags tests (cont...)"