Added clarity for installing Neutron plugin on dedicated controller

As reported to Launchpad, the instructions for setting up the Neutron
server on a dedicated controller that will NOT be providing any
underlying functionality are not clear as to why we need to install
and configure the plugin here.

Change-Id: Ia4ce1ea618116fa51e847e1bbb9b804ab4c32870
backport: havana
Closes-Bug: 1261158
Merge "Added clarity for installing Neutron plugin on dedicated controller"