Extracting auth methods into a new class

Extracting the authentication and authorization methods into
a new class in order to start introducing the new Keystone
backend.

Note: I'm proceeding in small tests to allow for fine grained
regression testing.

Change-Id: I719ea83ab67f97e5cd32d7fd3282b3caa47a65eb
Signed-off-by: Eduardo Sousa <esousa@whitestack.com>