Need to allow for a different private_key to be used in the `connection` block during provisioning

Per the Terraform documentation on provisioning, if we use a different private_key than the one that is typically used by ssh by default, I believe it will not work currently since we don't modify the private_key in the connection block. The Terraform docs are found here.

Just need to add this via the input ssh_key_file variable since that already points to the private key.