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
* update spec tests
* add release note
Change-Id: I2700421a187d8f815ba42a4160c46659876f854e
37ce8b45e6