Add support for image sync status updates via rabbitmq

- Adds separate status-listener script to connect to rabbitmq via kombu, and listen forever for status updates.
  - Writes status updates as text to a file ~/.cloud-install/sync-status (And logs errors to sync-status.error)
  - separate script is necessary because rabbitmq client stuff (kombu) is py2-only.

- Minimal gui changes, mostly to add a 'service info' field to each
  Node, to allow for info messages that apply to an entire
  service. This is then used to display the sync status.

- Adds status.py module with get_sync_status(), which simply launches
  the status-listener once and then reads the text file it writes.