Modify "ensemble status" to display exposed ports and whether a service is exposed or not

If a service has been exposed, then the ensemble status output is
suitably augmented. For the YAML serialization, for each exposed
service, the ``exposed`` key is added, with the value of ``yes``. For
each service unit of an exposed service with opened ports, the
``open-ports`` key is added, with its value a sequence of
``port/proto`` strings.

This will result in output similar to below::

  services:
    wordpress:
      exposed: yes
      formula: local:wordpress-42
      relations: {db: mysql, state: up}
      units:
        wordpress/0:
          machine: 2
          open-ports: [80/tcp]
          state: started