feat(vagrant): check if discovery url is defined

Check that `user-data` exists and that `make discovery-url` was executed before `vagrant up` (discovery is not commented)

It's required the [vagrant-triggers plugin](https://github.com/emyl/vagrant-triggers
 `vagrant plugin install vagrant-triggers`

```console
$ vagrant up
Bringing machine 'deis-1' up with 'virtualbox' provider...
Bringing machine 'deis-2' up with 'virtualbox' provider...
Bringing machine 'deis-3' up with 'virtualbox' provider...
==> deis-1: Running triggers before up...
Please run "make discovery-url" before "vagrant up"
```

closes #2342
Could we leave this section inside `if File.exist?(CLOUD_CONFIG_PATH)`? I know it's redundant since we checked up above, I'm just trying to minimize the diff with https://github.com/coreos/coreos-vagrant/blob/master/Vagrantfile.