Adapt configuration to fully support hybrid messaging backends

The oslo.messaging library has supported the separation of RPC and notification messaging backends since the Newton release. The current oslo.messaging configuration incrementally evolved over a number of releases and did not fully anticipate the requirements of hybrid backend deployments. As a result, operators face numerous configuration challenges as they take steps to move from a monolithic to hybrid messaging system deployment.

This blueprint proposes changes to the configuration that will allow for the independent configuration of the RPC and Notification backends (e.g. separate communication settings such as SSL). In addition, the new configuration will unify common driver option to improve the ease of use for operators looking to switch between different messaging backends. Finally, the ability to manage driver specific configuration options will be retained for advanced use cases.