GET v2.0/schemas/image(s)

Get schema that represents an image(s) entity.

Tested get image(s) schema
Added explicit assertion messages for all assertions as per reviewer comments.

Reordered the imports in the fixtures.
Moved MSG constant in the fixtures as per reviewer comment.

Moved msg constant to setUpClass in fixtures.

Added the validation of the returned image(s) schema in the tests.
Removed the custom assertion message.

Fixed string formating issues as per reviewer comments.

Change-Id: I431031b9ef7c30bfdcbe77e6445279bcfa2445af
Merge "GET v2.0/schemas/image(s)"