POST v2.0/images/{image_id}/tags/{tag}

Add tag to an image.

Tested add a valid tag to a valid image.
Refactored fixtures.

Added explicit assertion messages for all assertions as per reviewer comments.
Added a short description for each of the methods in the fixtures.
Made some small changes in the fixtures.

Moved MSG constant to fixtures.
Fixed line continuation issues.

Moved msg constant to fixtures setUpClass.
Fixed line continuation and import issues.

Removed the custom assertion message.
Added a new line.

Change-Id: Ife9274cd4899b9bf796eac08cab0118c39efa37e
Merge "POST v2.0/images/{image_id}/tags/{tag}"