The current default value of cinder::control_exchange equals to
the default value of [DEFAULT] control_exchange. This change makes
the service default used instead of redundant one hard-coded in
puppet layer.
Co-Authoered-By: Bogdan Dobrelya <bdobreli@redhat.com>
Change-Id: Ifd86d3ee30079d310269e7a954719246ec0dcdfd