Update control_exchange value in keystone.conf

The control_exchange comes from oslo and is set to 'openstack' by
default. This should be overridden by the projects that use
oslo.messaging, since projects should be placing notifications
on project specific exchanges.

Nova performs the override in the same manner:
https://github.com/openstack/nova/blob/master/nova/config.py#L33
https://github.com/openstack/nova/blob/master/nova/rpc.py#L77

Change-Id: Ifcc43dcccaa9d56e03922ce8cf0bf7fb8e93bdcc
Merge "Update control_exchange value in keystone.conf"