Remove default_notification_level from conf

This config option is used when you use the notify_wrapper wrapper
function from openstack.common. As we don't use the wrapper we don't
ever require the config option so we shouldn't show it to users.

blueprint oslo-messaging

Change-Id: I95280af37a84c71131fb2fc92c9302b8926c95bd
Merge "Remove default_notification_level from conf"