Add a note about restrictive firewall on rhel

RHEL/Centos/SL has a restrictive firewall by default, which
if left in an unaltered state prevents compute nodes from
talking to the controller (and therefore and unsuccessful
installation)

Since changing this is very basic sysadmin practice, just add
a small warning.

Change-Id: I5d564f612aaa6e7b14892bef79538dd3e387bfc9
Closes-Bug: 1241981
Merge "Add a note about restrictive firewall on rhel"