Added missing Neutron package install command for Ubuntu

In the Neutron installation documentation for a dedicated controller
node, there's missing entries for installing on a Ubuntu based
system.

Added the change for Debian also.

Change-Id: Ia30381eca87af273904f3608dc92f519a8ed6741
backport: havana
Closes-Bug: 1261155
Merge "Added missing Neutron package install command for Ubuntu"