DELETE v2.0/images/{image_id}/members/{member_id}

Delete a member from an image.

Tested delete a valid member from 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 to improve readability.
Refactored the fixtures to change the properties of some methods.

Change-Id: Ic7a6d5a6545531dbac1c15ef9cec66ea81aa7df9
Merge "DELETE v2.0/images/{image_id}/members/{member_id}"