Install Guide: Add missing Ubuntu instruction for Neutron install

The step was missing instructions for Ubuntu and Debian, add them.

Change-Id: I065323428c772a718ec45b312104c23db92a14d4
backport: havana
Closes-Bug: #1245750
Closes-Bug: #1241880
(cherry picked from commit 173be34bd22499a8a49aa3851915906d7d362cb7)
Merge "Install Guide: Add missing Ubuntu instruction for Neutron install" into stable/havana