rabbit: add some tests when rpc_backend is set
rabbit: add some tests when rpc_backend is set

This change improves test coverage of the rabbit driver for new
value allowed in rpc_backend config option.

Change-Id: I52a8a8beb5b0d32647f52bb21abd5f7164dac2df

apporc/oslo.messaging,hkumarmk/oslo.messaging,ozamiatin/oslo.messaging,magic0704/oslo.messaging,dukhlov/oslo.messaging,zhurongze/oslo.messaging,hkumarmk/oslo.messaging,dims/oslo.messaging,isyippee/oslo.messaging,apporc/oslo.messaging,stevei101/oslo.messaging,zhurongze/oslo.messaging,ozamiatin/oslo.messaging,isyippee/oslo.messaging,dukhlov/oslo.messaging,stevei101/oslo.messaging,magic0704/oslo.messaging,dims/oslo.messaging