Oslo.messaging uses the transport_url to represent the rpc and
notification messaging driver to use and its full configuration. The
rpc_backend configuration option is deprecated for removal and
should not gate oslo.messaging driver configuration options.
This patch:
* deprecate rpc_backend
* remove conditional check
* add release note
Change-Id: I00948197fef80ce54a9a382d251b54e39053f7a7