Refresh rpc_backend values in unit tests to those from oslo.messaging

Though old values are supported via compatibility transport aliases [1],
it's better to stick to those values that are officially supported by
oslo.messaging.

[1]: neutron/common/rpc.py:TRANSPORT_ALIASES

blueprint oslo-messaging

Change-Id: I7bc03e323151cfdda2cb2bc4cd6f5aea9b87a746
Merge "Refresh rpc_backend values in unit tests to those from oslo.messaging"