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

Part 2 of Images API v2.0 DELETE image tests

    blueprint glance-api-v2-tests

    Tests against the DELETE image functionality/endpoint of the Glance API
    V2.0.

    - remove custom assertion message
    - extract assertion message to fixture as constant
    - add assertion messages
    - add separate client to fixture to differentiate normal user and admin
      user actions.
    - improve assertion custom message
    - use defined constants for image types to remove string literals.
    - refactor 'test_delete_image_with_blank_image_id' for better readability.

    - remove patch set previous dependencies.
    - replace a shuffled invalid id to use a known invalid id.

Change-Id: Ia10c3b7ec5c832830d9b98fce2ba5e0ee438b6b6
Merge "Images API v2.0 DELETE image tests (cont.)"