According to https://docs.openstack.org/developer/oslo.messaging/opts.html#oslo-messaging-notifications a transport_url defined in [oslo_messaging_notifications] should be used, if present. In Ceilometer, this is ignored in favour of the transport_url defined in [DEFAULT], which is a bug. This change fixes the bug. Change-Id: I12a559ebbbc1edd18c6833943bb5e9af7adf4666