Use squid chache defined via WPAD if available

Test whether network makes WPAD configuration avaiabale. Set cvmfs to
use squid auto discovery if WPAD is present, else install and use shoal.

Changes:
	modified:   modules/cvmfs/templates/default.local.erb
	new file:   modules/gce_node/lib/facter/use_wpad.rb
	modified:   modules/gce_node/manifests/atlas.pp