Use shoal to find nearest cvmfs cache

Install, configure, and run shoal to allow dynamic squid proxy
configuration on the VMs as they boot.

Changes:
	modified:   .gitignore
	modified:   gce_node/manifests/init.pp
	modified:   packagerepos/manifests/init.pp
	new file:   shoal/files/shoal.repo
	new file:   shoal/manifests/init.pp
	new file:   shoal/templates/shoal_client.conf.erb