Allow to omit OVPN_NATDEVICE in iptables command

[john8329]

In my use case I'd like to NAT to all the available interfaces without having to specify them. Currently if I leave the option blank it defaults to 'eth0' in ovpn_run:67.

It would be useful to:

• Have the option documented better
• Edit the option so it can be omitted, this way iptables isn't limited to one interface only

[john8329]

By the way, a workaround at the moment is specifying an environment variable as OVPN_NATDEVICE=eth+

[larssb]

@john8329 I think the workaround is enough. I have no ties to the project though. However, consider a PR for this - docs on the workaround.