fix(Vagrantfile): disable replacing of default insecure key

In Vagrant 1.7, using the default `~/.vagrant.d/insecure_private_key` now triggers the generation of a different key per node as a security feature. This breaks our vagrant workflow since `ssh-add ~/.vagrant.d/insecure_private_key` will not provide auth for `deisctl`. We should also document how to add a specific private key to a `config.rb` file for a more secure setup, but this will fix things for now.

Thanks to @bcwaldon for pointing out this fix in coreos/coreos-vagrant#192. (We try to minimize the diff between Deis' Vagrantfile and that one, but we don't maintain a proper fork right now since it led some users astray from the main project.)

Closes #2727.