PATCH v2.0/images (cont...)

Update a specified image under different scenarios.

Tested different scenarios for image update.
Added explicit assertion messages for all assertions.
Fixed line continuation issues.
Refactored fixtures - reordered imports and added MSG constant.

Removed the custom assertion message.
Refactored fixtures to change method properties to be non class method.

Made some minor changes as per reviewer comments.

Change-Id: I2fc9109468b585ecb87e26613bd88d1046e347f7