This change adds the ability to listen for events across multiple
rabbit queues. Nova cells v2 has a message broker per cell and
the charms already support having a separate message broker for
neutron, in both these topologies ceilometer needs the ability to
listen to multiple brokers.

To achieve this a new relation 'amqp-listener' has been introduced.
The existing 'amqp' relation should be used for the broker that
ceilometer listens to and publishes to. 'amqp-listener' should be
used for additional brokers that ceilometer just listens on.

Update functional tests to satisfy relation with
nova-cloud-controller.

Change-Id: Ifdade3f7814620f4cd4a1d35a584cbc099bb6d88