Create machine

This adds the ability to select the constraints before creating a new machine.

A continuation of https://github.com/juju/juju-gui/pull/366

QA:
- open the machine view
- click "Add machine"
- a constraints form should appear
- if you wish fill in some constraints
- click create
- a new machine token "new0" should appear

To test cancel with unplaced units:
- drag a service to the canvas
- open the machine view
- drag the unplaced unit to "Create new machine"
- the unplaced unit token should have been removed and a constraints form should appear
- click cancel
- the unplaced unit token should  reappear
We should probably discuss this more. I tried switching to `container.empty()` but found that destructors on child views (i.e., `service-scale-up-view.js`) would fail because their containers no longer had a DOM node.