Use transport_url instead of rpc_backend
Use transport_url instead of rpc_backend

oslo.messaging has removed rpc_backend in commit
d3f6ca0b47e2a5a3413b27617d9655f02b191c6a, use transport_url instead.

Change-Id: Ic51c79c0c0a1338e7a8fab998ceb6413de18920a
Closes-Bug: #1797781

openstack/python-tripleoclient