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

Register VM image under different scenarios.

Tested different scenarios for image creation register.
Added explicit assertion messages for all assertions.
Refactored fixtures, reordered imports.

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

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

Fixed new line continuation issues as per reviewer comments.

Removed the custom assertion message.
Corrected the branch name.

Fixed line continuation issues.
Refactored fixtures to change method properties.

Fixed format string constant issues as per reviewer comments.
Fixed rebase conflict issues.

Change-Id: Ied1e77802707579694a3e3ecce0106f7b513d784
Merge "POST v2.0/images (cont...)"