Fix management-external-cert option parsing error

Fix management-external-cert option parsing error

 - Allow --management-external-cert as an alternative to --cert
 - Also make sure --cert and --management-external-cert are not
   both specified, and clarify in the man page that the latter
   must be used with --management-external-key.

Signed-off-by: Selva Nair <selva.nair@gmail.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1466132093-1178-1-git-send-email-selva.nair@gmail.com>
URL: http://article.gmane.org/gmane.network.openvpn.devel/11929
Signed-off-by: Gert Doering <gert@greenie.muc.de>