Support for VPN

[mcapuccini]

As per discussion in Trygve2 meeting, it could be a good idea to add support for VPN gateway in KubeNow. It should be relatively easy, as cloud providers and Terraform have native support for that:

https://www.terraform.io/docs/providers/aws/d/vpn_gateway.html
https://www.terraform.io/docs/providers/google/d/datasource_compute_vpn_gateway.html
https://www.terraform.io/docs/providers/azurerm/d/virtual_network_gateway.html

I think that only OpenStack doesn't support this.