openvpn: Allow override of interface name

openvpn: Allow override of interface name

If using a configuration file for OpenVPN, allow overriding name of the
interface. The reason is that then people could use configuration file
provided by VPN provider directly and override the name of the interface
to include it in correct firewall zone without need to alter the
configuration file.

Signed-off-by: Michal Hrusecky <michal@hrusecky.net>
(cherry picked from commit c93667358515ec078ef4ac96393623ac084e5c9e)