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
(cherry picked from commit 63a0babcf38c3c0d824a4e54c4c89f5c1658c48a)
correction to text in manual

Change-Id: I669a0ce20257ffa4ce1ad13e81306e56e61e91bf
Closes-Bug: #1241050
Partial-bug: 1241050
(cherry picked from commit 51ac892c4ff25822c0acff452eae160493b80972)
Merge "Add step for "Compute and Network Node - L2 Agent" install  instructions" into stable/havana
Merge "correction to text in manual" into stable/havana