openvpn: enable management interface

This allows https://github.com/brianhornsby/script.openvpn to work as a
openvpn configuration interface. Once this has been road-tested we can
remove currently-broken openvpn support from connman and OE settings.
Merge pull request #3208 from chewitt/openvpn_mgmt_interface

openvpn: enable management interface