Update a machine token when units are added to its containers

Fixes: https://bugs.launchpad.net/juju-gui/+bug/1342861

Adding units to a container should update the icons on a machine token.
I think that this method is in need of some commenting. I'm confused that the `_updateMachineWithUnitData` method can be called twice with....different data? Below machineToken...may... be overridden if there is a container token? 

Anyways, it's just not clear why this method is doing what it's doing so it would be very helpful if you could detail the 'why' of the execution flow of this method.