Ensure proper committed status on containers.

This fixes bug https://bugs.launchpad.net/juju-gui/+bug/1369601

Containers had been (incorrectly) inheriting their commited status from
their parent machine. That meant that a newly-created container on an
existing machine would show up as committed upon re-render. We now check
for 'new' in the name of each container and set its committed status
based on that. The only exception is root containers, which still
inherit from the parent machine because they *are* the parent machine.
The message is displayed when the assertion fails, so in this case the new container, which should be uncommitted, is not.