Linked Neutron overview content in the Installation Guide

'xi:include parse' the  neutron overview content in the OpenStack Networking section

backport: Juno
Closes-Bug: #1340524

Change-Id: Ifaf92418552396c992764cb55696da30dd654853
(cherry picked from commit 25c96dd7e552de26b80343066e5cc32a9f5fce50)