Cleans up unplaced service unit code.

This is a trivial branch ladning some minor refactors to the unplaced service
unit code.

* Service unit token rendering is broken out into its own method, both for
  consistency with other tokens and to work more cleanly with _smartUpdateList
* The unused machine and containers attr from the service units has been
  removed; containers was never populated and neither were used, nor appear to
  need to be used.

If I'm wrong on the last assertion I'll happily revert that change.

Service unit rendering is well tested; as this just moves functionality out of
an anonymous function and removes unused and untested bits, no new tests are
needed.