Add systemd unit file for OpenVPN

This is to encourage all Linux distributions to use a unified systemd
unit file.

This unit file also tries to reduce the capabilities of the running
openvpn process.

Signed-off-by: David Sommerseth <davids@redhat.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: 1411030936-16309-1-git-send-email-openvpn.list@topphemmelig.net
URL: http://article.gmane.org/gmane.network.openvpn.devel/9043