explicitly set rpc_backend in nova.conf

Implements: blueprint centos-rhel-for-compute
Fixes: bug #1269077

The following is a default value in openstack:

rpc_backend=nova.openstack.common.rpc.impl_kombu

Using RDO (rhel/centos) packages, this gets overridden in
/usr/share/nova/nova-dist.conf to 'impl_qpid'.

In order to cleanly set this back to the default value (which is what we
want as a default in the cookbooks), we need to explicitly set this in
nova.conf.

Change-Id: Idd025034c07ce12810a38eff87975b1c8caf1985