Structured and qualified notifications for oslo.messaging

Notifications API in Oslo is currently very generic and doesn't many structuring fields on the payload sent. That doesn't help engine that would like to store, index and analyzes the event notifications. Adding more documented fields that should/mut be present in notifiations could help a lot in this regard.

Decouple cfg handling out of oslo.messaging for oslo.messaging

The goal of this blueprint is to be able to use oslo.messaging without using a configuration file/object, while keeping its usage possible and not breaking compatibility with OpenStack applications.