BZ#1168985 - systemctl start openstack-nova-compute failed

https://bugzilla.redhat.com/show_bug.cgi?id=1168985

nova-compute service couldn't communicate with nova-conductor and
failed to start. It seems that the default values for control_exchange
in nova-conductor and nova-compute didn't match each other, causing
them not to find each other's messages.

Set both explicitly to get rid of the inconsistence.