Pass the correct tenant for the service user

Pass the correct tenant for the service user

Previously, the service user (which authenticates tokens passes to the
engine by the APIs) was paired with the tenant of the end user to do
authentication, which worked only when they were in the same tenant. This
should not be the case, since the service user should only have an admin
role in the "service" tenant.

Fixes #269

Change-Id: I99b764fc489429dd6d081485bd89d5dde737c124
Signed-off-by: Zane Bitter <zbitter@redhat.com>