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'm +1 on returning the original error if there's only one failure, but I think we need to be consistent with the error output.

I think it might be better just to print out the "failed to create N machine(s)" regardless of how many there are, and  then return cmd.ErrSilent. Also, I think you want to be printing to ctx.Stderr() for errors -- they shouldn't be hidden by `-q` IMO.