openvpn: enable management interface

This allows https://github.com/brianhornsby/script.openvpn to work as an openvpn configuration interface with OE. Once this has been field-tested we can remove the horribly-broken openvpn support from the OE settings addon. In ref: https://github.com/brianhornsby/script.openvpn/issues/13#issuecomment-42759934