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.

Note, this is a forward port to avoid problems caused with change
Ia4ce1ea618116fa51e847e1bbb9b804ab4c32870

Change-Id: I609d187321c2e0ff82f45fe24118f5b31bef7791
(cherry picked from commit 91711b4350f240d0a12a7f68b6b09c40045e73a6)
Merge "Added clarity for installing Neutron plugin on dedicated controller"