Add documentation for new role debops.openvpn

[htgoebel]

See my new role which I like to contribute at https://github.com/htgoebel/ansible-openvpn

[drybjed]

About firewall limitation: I've encountered similar case with PostgreSQL, where you can configure multiple clusters on different ports. I ended up managing the ferm configuration file "manually" from the debops.postgresql role and just notifying ferm to reconfigure the firewall, which is OK and supported by debops.ferm. You might want to consider registering additional ports in /etc/services via debops.etc_services role (which is also done "manually" by the debops.postgresql role).

About SSL certificates: new debops.pki role can handle that. You could try creating your own PKI authority and realm (via variable dependencies). More information about how to test this role can be found in the post on the mailing list. Currently there's no documentation, so let me know if something is unclear.

[ypid]

@htgoebel I think you can close this PR and reopen using the new docs format against debops-contrib/docs after we solved #111.