POST v2.0/images/{image_id}/members.

Add a valid member (tenant_id) to a valid image.

Tested add a valid member 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.

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.
Corrected the branch name.

Added a new line.
Refactored the fixtures to change the properties of some methods.

Change-Id: I594253033f3ce9dd88badd2a2cc3e664228fa0ee
Merge "POST v2.0/images/{image_id}/members."