Fixed drag and drop of unplaced units

Drag and drop of unplaced units was broken. This fixes it.

Bug: https://bugs.launchpad.net/juju-gui/+bug/1323199

QA:
- drag a service to the canvas
- open the machine view
- drag the unplaced unit to 'create a machine'
- open the javascript console and check there are no errors.
- click deploy and confirm
The unit should be on a new machine. Adding more units and dropping them on existing containers should work as well.