Move _BaseController to common/controllers.py

The _BaseController class from the Federation work offered
some nice functionality for ensuring the controller checks for required
arguments in resource CRUD. Moving this to controller.V3Controller so
that it can be used consistently on resources in the Identity V3 API.

Related-Bug: 1278739
Change-Id: Icb66e2e324820959fa9409a3b1388c89b82f8ec0
Merge "Move _BaseController to common/controllers.py"