Fixes neutron setup for Debian

This patch completely removes the section_neutron-install.xml
section from the Debian install-guide, since that part is
completely automated in Debian, and describes how the plugin
system works in Debian. It then customize the other sections
to fit the Debian installation procedure.

Change-Id: Ib1fabd7f64d513132484b98541cef2e4276ccfe8
backport: havana
(cherry picked from commit ac32f3ac9482eeeb3263bee51c097cb040eda0b5)
Merge "Fixes neutron setup for Debian" into stable/havana