Add step for "Compute and Network Node - L2 Agent" install  instructions

Adds step, formats it for rhel, centos, fedora

Creates a symbolic link from /etc/neutron/plugin.ini to /etc/neutron/plugins/openvswitch/ovs_neutron_plugin.ini or neutron-server will not run on Centos

Change-Id: I5f5e31f315237d6eeb6a51d28e3dc799081a43a7
author: nermina miller
backport: havana
Closes-Bug: 1240697
Merge "Add step for "Compute and Network Node - L2 Agent" install  instructions"