PUT v2.0/images/{image_id}/file.

Register and upload new image.

Tested upload image with an image file.

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.

Reordered imports in the fixtures as per reviewer comment.
Moved MSG constant to fixtures.
Fixed line continuation issues.

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

Removed the custom assertion message.
Added a new line.

Postponed the implementation of the smoke test. It requires implementation of new functionality.
Refactored the fixtures to change the properties of some methods.

Change-Id: Ifc963fb8086b393e69b58b5591d701a6e8e35ba2
Merge "PUT v2.0/images/{image_id}/file."