Don't create machines that don't exist.

**QA:**

This must be done in a real environment, in machine view.

1. Create a new machine by clicking on the "Add machine" link in the header.
2. Select a charm in the charmbrowser and click on the "Add to canvas" button. You should see a unit show up in the unplaced units column.
3. Select the newly created machine (still undeployed) so that the bare metal container is visible.
4. Drag and drop the unplaced unit on the bare metal container.
5. Deploy everything and wait a few seconds. You should get an error message (due to another bug).
6. The machine view should re-render with the unit back as an unplaced unit. The newly-created machine should now be deployed, but with nothing on it. There should *only* be one machine created. Before fixing this bug, two machines would show up in the column, one with no ID and showing the unit as being deployed, like so: http://cl.ly/image/0y2o032x2v1Z