Remove possibility of using --tls-auth with non OpenVPN Static key files

In older version OpenVPN would hash a --tls-auth file
if it does not conform to the expected format
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <1417871704-30273-1-git-send-email-arne@rfc2549.org>
URL: http://article.gmane.org/gmane.network.openvpn.devel/9306

Signed-off-by: Gert Doering <gert@greenie.muc.de>