lib/facter/sensors.rb, only run the plugin for physical machines, by checking…

See merge request !1

lib/facter/sensors.rb, only run the plugin for physical machines, by checking the virtual-fact=physical first

init.pp, allow blacklisting certain sensors by placing their name into /etc/facter/sensors_backlist.conf to avoid getting them picked up by sensors.rb

sensors.rb, in case the sensors-binary can not be found, log a warning instead of throwing an exception