Migrate to override_role for identity v2 module

Now that override_role has supplanted switch_role (which has
been deprecated) in [0], the RBAC tests need to switch to use
override_role.

This PS switches to override_role for the identity ve module.
This patch handles all the remaining modules for identity v2.

[0] I670fba358bf321eae0d22d18cea6d2f530f00716

Partially Implements: blueprint rbac-utils-contextmanager

Change-Id: Ia8c0a465e906239e1572fdda2eceea820328faa8
6 files changed