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
Not sure I follow… the "We no longer use relation_errors…" comment (and the skipped test) isn't mine. This was just one of the tests I moved inside the describe block. Unless I'm misunderstanding?