Nova uses the Qpid AMQP driver by default as the RPC backend, but Quantum uses the Kombu (RabbitMQ) driver which is the upstream default. The Quantum default should be changed to Qpid also, since that is the supported AMQP implementation.
Indeed, quantum.conf distributed in RPM doesn't set rpc_backend, so cfg default applies, quantum.openstack.common.rpc.impl_kombu But quantum-{node|dhcp|server}-setup scripts do set rpc_backend to quantum.openstack.common.rpc.impl_qpid along with qpid_hostname. Anyway, I agree we should make configs consistent out of the box.
*** This bug has been marked as a duplicate of bug 887822 ***