Prevent unit/service deletes from cascading to machines.

Prevents cascading deletes from happening to containers/machines when a unit/service they host is deleted. Fixes these bugs:

https://bugs.launchpad.net/juju-gui/+bug/1368556
https://bugs.launchpad.net/juju-gui/+bug/1368573
https://bugs.launchpad.net/juju-gui/+bug/1368578
New test to verify that we always create when a change event comes through for a model that doesn't yet existing in the DB. This functionality was already in place, but per @hatched I added a test around it to ensure we don't unknowingly change this in the future.