Images API v2.0 Get Image File Tests

Implements new tests for get image file scenario:
 - added a new test module 'test_get_image_file_negative.py'
 - added a new test scenario in 'test_get_image.py' module.

Refactors the tests:
 - removed setUp class methods
 - added image creation, basic image data file in the fixtures
 - added a '_upload_image_file()' method.

Rebase change and upload a new patch set.

Refactors fixtures and tests:
 - put back set up classes in tests
 - removed image creation and moved it back to test set up classes
 - moved the '_upload_image_file()' method to fixtures

Change-Id: I36a197fb6fa40a83eb62ab4860dd2e31156bc358
Merge "Images API v2.0 Get Image File Tests"