Add sysctl for network optimization

Tune the network for high data trhoughput according to the parameters
suggests by ESnet fasterdata for linux tuning. This is assuming we will
get rates of 155 Mbps or greater. Introduces dependence on thias/sysctl
module from puppetforge.  Install it on puppet master via:

puppet module install thias/sysctl

Changes:
   * modified: gce_node/manifests/init.pp