status output changes

[William Reade]

lp:871743 and lp:769120 imply changes to status output, and we should
make sure we've considered them properly before we actually make the
changes. So, I propose that:

1) Unit state dicts' "state"s will contain the following possible
values:

  * If no unit workflow state has been set: "pending"
  * Elif unit agent is not connected: "down"
  * Else: the current value of the unit workflow state

2) Machine state dicts' "state"s will contain the following possible
values:

  * If a machine agent is connected: "running"
  * Elif every assigned unit is "pending": "pending (instance: %s)"
  * Else: "down (instance: %s)"

When a machine agent is not running, the "(instance: %s)"s are intended
to give users visibility into the underlying state of the provider
machine; it's useful to know, for example, the difference between an
orchestra machine that was never successfully provisioned as opposed to
one that "should" now be running.

Does anyone have any enhancements/objections?

Cheers
William