always use Vagrants insecure key

Vagrant 1.7 introduced a new feature that replaces the publicly known key with a newly generated one [[1]](https://twitter.com/mitchellh/status/525704126647128064). This fails with more than one VM (the key will get updated only in the first VM).

Arguably that is a bug in Vagrant but since it shouldn't matter for the use case of this box the practical solution is to just disable this feature...