Extend snf-dispatcher callbacks with one to run every time an
OP_CLUSTER_ opcode is executed in a Ganeti backend.

* make snf-ganeti-eventd send messages on cluster modifications. These
  messages do not contain any description about the job.
* create new queue %(prefix)-events-cluster with routing key
  'ganeti.event.cluster'.
* add update_cluster callback to update the disk templates and the
  resources of the backend.