Warn user about unsupported API version
Warn user about unsupported API version

Prints a warning when the user tries to use an unsupported API.
Also provides a test case to check it still does fall back to 2.0.

Change-Id: Idabd150f52d27a9d065cf570f6367459126f98be
Closes-bug: #1256987

varunarya10/keystonemiddleware