Comment 7 for bug 1761405

I feel that our current knob oslo_messaging_notifications.driver = noop is enough to turn off the notifications. But I agree that this needs to be documented a bit better in the notification dev ref and in the description of the notification_format config option.

@Matt (comments#3): Actually nova skips the expensive part of the versioned payload generation if the driver is set to 'noop' [1][2]. So I think we are OK to just document the possible config a bit better.

[1] https://github.com/openstack/nova/blob/06ad7dffa60dc4b38be3ca1b44db4bfb2f4c052c/nova/rpc.py#L218
[2] https://github.com/openstack/oslo.messaging/blob/393ecff3451091404832dd6b8a088e1bec760101/oslo_messaging/notify/notifier.py#L430