Destroy machine when last principal unit removed.

This change causes a machine to be automatically removed when the last principal unit running on that machine is removed (remove-unit). If the machine is a parent to any containers, it is left alone.

Background:
https://bugs.launchpad.net/juju-core/+bug/1206532
https://bugs.launchpad.net/juju-core/+bug/1183309/comments/8

This could use more test coverage, but I'd like to get a general feel for how it's looking so far before I go much further with it. This is my first time mucking around with state transactions. Please let me know if I've got it right!

I haven't added an option to keep empty machines/containers around after removing the last unit. I'd like to see if we can avoid it. If we do need the option, I think it should be opt in. Let me know.
I think the cleanupForceDestroyedMachine is the part that we probably need to be careful with.

I can see a point of "if we've done our early checks correctly, then it should be fine to force destroy, because we've already asserted everything is gone". But it feels a bit like "if we make a mistake in preconditions, we will just nuke everything anyway".