Allow add-machine to take a -n param

Fix for https://bugs.launchpad.net/juju-core/+bug/1214209

Allow add-machine to take a -n param to add multiple machines
I don't think we want to error out so early. If we've successfully created a machine, I think the user should know about it. How about saving the errors until the end, so you get something like:

    created machine 1
    created machine 2
    failed to create 3 machines:
      - error1
      - error2
      - error3

(and some kind of combination error returned)