Added section for Compute node firewall config

Added a new section to the Config Reference on what ports to open on
each compute node (and how). Upon consultation, it seems that adding
a port config section in each component chapter is a better way to
address bug#1261617.

I'll submit follow-up patches adding a firewall configuration section
to each Config Ref Guide chapter/component if this patch gets merged.

Change-Id: Ie53e73b7cd2ec7af81f264338a4676952dd168e1
Partial-Bug: #1261617
Merge "Added section for Compute node firewall config"