Migrate to override_role for volume module (last)

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 remaining one
volume file.

Other volume files are already taken care till
I3d134db7efb0717546c224c8e53ff687984d47d1

[0] I670fba358bf321eae0d22d18cea6d2f530f00716

Partially Implements: blueprint rbac-utils-contextmanager

Change-Id: I4590971e39e54e8cf7e13c076e3e2a9647888695
1 file changed