Destroy container units

Added functionality to remove units that are on container tokens.
It actually does get a deleted attribute already. I use this when re-rendering the units on the container to display the deleted state (see: https://github.com/juju/juju-gui/pull/538/files#diff-c9f5f14812916b5e818b644e0ec59c3cR2), assuming I'm understanding your suggestion!