Image API V2.0 DELETE tags tests

Implements new test scenarios for 'test_delete_tags.py'
- add more test cases to DELETE image tags
- change test_delete_invalid_tag_from_valid_image to test_delete_invalid_tag_with_special_characters_from_valid_image
- refactor test_delete_multiple_tags_from_image to return empty tags list

Change-Id: I7c305664456fd2f9526b569ae5cdb77cf745a70b
Merge "Image API V2.0 DELETE tags tests"