You can now drag and drop an unplaced unit on the container header and it'll create a ghost container

#### To QA
- Drag and drop the mediawiki:single bundle
- Once it's deployed drag and drop postgres
- Switch to the machine view
- Click on the first machine so that it shows the containers
- Drag the unplaced unit onto the container columns header
- You should now have two containers, one an lxc with postgres and the other from the bundle.
- Click commit to make sure it all commits properly.