Change I5ff9c4e4b6d64750f5db2a73cc4317358aea0649 restructured the
identity subsystem. As part of the change, the abstract driver
was extracted to identity/backends/base.py
This change does the same for the endpoint policy subsystem.
Change-Id: Ia7fb2b49cdbe75831a971cce3f33f0cfa269816b