Changes for using the plugin in RHEL / CentOS

I modified the code to use both Debian + RedHat systems with the plugin. The difference is in munin node log path - where Debian is using /var/log/munin and RedHat is using /var/log/munin-node. If I use the current plugin in CentOS, it will not start, because log_file in munin-node.conf.erb pointing to the /var/log/munin/... which doesn't exist in CentOS.