Support for image sync progress status updates via rabbitmq

- NOTE: this adds ppa:cloud-installer/simplestreams-testing to get an
  unreleased version of simplestreams built for trusty that has
  support for the progress API we use here.

- Add support for a relation to something that provides amqp (tested
  with rabbitmq-server)

- Defines StatusMessageProgressAggregator, which uses the
  simplestreams ProgressAggregator API to build a useful status
  message and send it via a given callback

- If a rabbit connection can be made, sends status messages to a
  rabbit queue for anyone to grab. Falls back to happy silence on
  error.

- If the simplestreams API is also there, status messages include
  per-file and total info. If the API is not there for some reason,
  but rabbit still works, the script falls back to just showing
  "starting" and "done".