Vagrant supports multiple private keys and take "private_key_path" as an Array

Latest Vagrant takes "private_key_path" as an Array.

config.ssh.private_key_path - You can also specify multiple private keys by setting this to be __an array__. This is useful, for example, if you use the default private key to bootstrap the machine, but replace it with perhaps a more secure key later.

I made this patch to be compatible with old and new version of Vagrant.