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
XXX is just a string we can use to grep for issues we need to revisit in the codebase