Add check for RABBIT_HOST and RABBIT_PASSWORD in iniset_rpc_backend
Add check for RABBIT_HOST and RABBIT_PASSWORD in iniset_rpc_backend

In multi-node environment, RABBIT_HOST and RABBIT_PASSWORD are used to
indicate the server running rabbitmq service. We should check the
variables in iniset_rpc_backend.

Change-Id: Iaea8cc87315be91429a8747254310d6474930eec

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