openvpn: Add OpenVPN support

* Add OpenVPN support to overcome the annoying NAT transversal issues.
  The intention here is that users have some Tahoe storage VPN that all
  the storage nodes and clients have access to.
* User supervisord to run the show.
* Leverage the /tahoe volume for persistent data.
docs: Add debugging document

* Add document while I'm thinking of it to save a lookup or two later.