Build crash if --openvpn NOT specified

[darkpixel]

INFO:root:Building package for router: router1
chmod: missing operand after 400' Trychmod --help' for more information.
Traceback (most recent call last):
File "/home/aaron/.virtualenvs/omtest/bin/openmesher.py", line 154, in
main()
File "/home/aaron/.virtualenvs/omtest/bin/openmesher.py", line 142, in main
plugin.plugin_object.process(m, include_dirs=includedirs, restart_services=includedirs, configPlugins=configPlugins, cliargs=arg)
File "/home/aaron/.virtualenvs/omtest/lib/python2.7/site-packages/OpenMesher/plugins/makedebs/init.py", line 96, in process
raise Exception('Package generation failed in %s. Do you have debhelper and fakeroot installed?' %(base_path))
Exception: Package generation failed in /tmp/openmesher-nrRPK2. Do you have debhelper and fakeroot installed?
(omtest)19:56 ~ $

This fails due to the following line in debian/rules:
find -name "*.key" | xargs chmod 400

If --openvpn is not specified, don't try to chmod non-existent key files.