systemd: Enable systemd's auto-restart feature for server profiles

systemd: Enable systemd's auto-restart feature for server profiles

Systemd supervises services it has started and can act upon unexpected
scenarios.  This change will restart OpenVPN after 5 seconds if the OpenVPN
process exits unexpectedly.

The on-failure mode is the recommended mode by upstream systemd.

This change have been tested on a test server for some month, and it
works indeed as intended when provoking the OpenVPN process to stop.

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: David Sommerseth <davids@openvpn.net>
Message-Id: <20170906235202.26551-1-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg15370.html
Signed-off-by: David Sommerseth <davids@openvpn.net>