Set oslo_messaging_notifications driver
Set oslo_messaging_notifications driver

This config option is requied in order for nova notificaions to
function, and enabling it doesn't cause any harm (there is another
option for turning notifications on).

Change-Id: I309af6cc43af485f795c368d304ebe71fceb1a03

group-policy/devstack,group-policy/devstack,dtroyer/devstack,nawawi/openstack,nawawi/openstack,bswartz/devstack,richliu/devstack,noironetworks/devstack,bswartz/devstack,openstack-dev/devstack,richliu/devstack,noironetworks/devstack,dtroyer/devstack,openstack-dev/devstack