Auto start Openvpn with one .ovpn file
When you have installed openvpn from the repo, everything is already prepared for you.
Place the whatever.conf file in /etc/openvpn/ and a daemon will be started for it at boot.
Edit
Please note that *.opvn will not work, *.conf will.
Note, that /etc/default/openvpn file contain AUTOSTART option. You can add there all your configs without .conf at the end you want to autostart. But don't forget to do:
systemctl daemon-reload
after editing it.