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 that reproduces the bug. Verifies that we process a "change" delta for machines when a unit is removed, instead of just percolating the unit's "remove" delta on down.