Migrate to override_role for volume module (part 3)

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 volume module. This
PS handles the remaining modules for volume.

This PS also removes unnecessary indexing into response bodies.

[0] I670fba358bf321eae0d22d18cea6d2f530f00716

Partially Implements: blueprint rbac-utils-contextmanager

Change-Id: I7451b7b098b9f751af777fcfad947ebc7a72f5ae
11 files changed