Generalize control_exchange attribute

The control exchange attribute was placed under 'rabbit' but it is
actually a property for 'qpid' too so moved it out at the same level as
the related 'rpc_backend'. Also moved it closer to rpc_backend in the
template.

Documented both attributes.

Change-Id: Iade5a48abbb865cf3a21c9beb8e6cc7783517fb0
Merge "Generalize control_exchange attribute"