Do not ask vagrant to generate a per-vm private key

Since Vagrant 1.7, vagrant auto-detects whether the vagrant insecure key is
used and automagically replace it with an generated key at
.vagrant/machines/<machine name>/<VM Manager>/private_key.

This option makes vagrant skip the insecure key detection (see
https://twitter.com/mitchellh/status/525704126647128064 for the ref)