Migrate to override_role for image module (part 1)

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 4 files.

[0] I670fba358bf321eae0d22d18cea6d2f530f00716

Partially Implements: blueprint rbac-utils-contextmanager

Change-Id: I91160f8aa8c1831b33dcaa866104a73403293354
4 files changed