support for changing source addr in vpn

[zestrada]

This PR closes #498 , adding a feature to vpnguin where users can specify a source address for services that expect a particular source ip. We accomplish this by configuring one of the dummy devices on the system with that address and then have the client side of vpnguin bind to that address.

We use the guest_cmd feature to stand up the interface if it is not configured.

Example usage:

plugins:
  vpn:
    depends_on: core
    spoof:
      "tcp:192.168.1.1:5678":
        source: 10.10.10.1
        dev: eth1
      "udp:192.168.1.1:12345":
        source: 10.10.10.1
        dev: eth1

Tested with TCP and UDP services that process the incoming source address, as well as various firmware that we've seen stand up multiple network services.

Note: this PR should be merged after rehosting/vpnguin#19 , with the correct VPN_VERSION in the penguin Dockerfile. The current VPN_VERSION in this branch is a guess and build will fail for now.