Restore "openvpn@.service" from packaging of openvpn 2.3.6, so we can
start the sysmocom vpn the same way as currently described in the
manuals.
Previous patch 8439e71a99 already
attempted to do this, but only brought back "openvpn.service" and not
"openvpn@.service".
Fixes: 8439e71a ("openvpn: bring back openvpn.service file")
Related: SYS#6303
Change-Id: I28a7f491d74690409f815555743679c69e31b518