Cleanup the toggling of the unplaced units message

This is a small branch to clan up the toggling of the unplaced units message. See my comment on PR #300 for the original branch.

#### To QA
- Use il/mv.
- Open the machine view, the unplaced units message should be visible.
- Click the + and the subsequent x for the mass scale up UI and the message should hide/show appropriately.
- Switch to the service view and drag a charm onto the canvas.
- Switch back to the machine view, the message should be hidden.
- Toggle the mass scale up UI, the message should not appear.
- Drag the service to the new machine header and then click Deploy/Confirm - after a bit the unit token will disappear and the unplaced message should show up.
- Close/open the machine view and toggle the mass scale up UI, the message should show/hide appropriately.