Add ability to configure control_exchange

Add control_exchange parameter with default to 'openstack'.

The control_exchange configuration defaults to 'openstack' in Cinder.

Fixes: bug #1220272
Change-Id: Ic15e471a745568e5183ad9eedba92c213026309e
(cherry picked from commit 15e32b1e4ab513fbe980e84d6585be42385c0ca7)