Migrate to override_role for image module (part 2)

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 image module for
remaining files.

[0] I670fba358bf321eae0d22d18cea6d2f530f00716

Partially Implements: blueprint rbac-utils-contextmanager

Change-Id: I971c1a24858a9a505257402ce3bf3cceaf7648b7
3 files changed