In the oslo.messaging to enable or disable notifications
uses the 'notification_driver' option. This approach used
in many Openstack projects.
By default the option is empty, that means no notification will be send.
To enable notification needs to set the value with one of supported drivers
(e.g. messagingv2).
Change-Id: Ifdc38d56a4fa3a1755fff72d4bef88937c0170ed