Fix error message when using RHEL init script

Fix error message when using RHEL init script

In RHEL 7 /etc/sysconfig/network is no longer used (still there but
empty). This results in the following error when openvpn starts:

Dec 20 09:01:25 localhost openvpn: /etc/rc.d/init.d/openvpn:
  line 94: [: =: unary operator expected

Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <c042fd01f62d707477c37e0298e303f1@vdberg.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18057.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 7e711035f12a25199c3a04193ee4e22e43930f6a)