Find and select the machine token on drop

* New method to find a machine or container token given the machine's name
* Update call sites to select the machine/container token on creation.
* Tests.
On second thought, is querying the DOM the best way to find this info? We maintain internal lists of all the tokens, keyed on their IDs:

~~~~
var machineToken = this.get('machineTokens')[name];
~~~~

That will return an instance of the corresponding token class. It looks like right now we're passing them into the `select[Machine|Container]Token` function, which needs the instance's container, not the instance itself. Still, that way we avoid DOM overhead.