Remember which interface was used by each OpenVPN conf

When interfaces go down and up we need to know which interface (vr1, vr2 etc) each OpenVPN instance is using so we can optimize our decision about which instances to resync. That data is not in the conf file (the conf file contains the IP address the instance binds to). This change puts the interface name into a little file in /var/etc/openvpn for later use.