Config ref: document options modified in icehouse

The tables of modified/new/deprecated options is automatically generated
using the diff_branches.py script (https://review.openstack.org/97620).

This commit handles the havana->icehouse changes to be easily backported
the stable/icehouse. It can be updated for each milestone to document
the latest changes.

Change-Id: I4821be3ce72b2ab69b81e89d5584334b77e7a098
backport: icehouse
(cherry picked from commit 2621bcb92f43ca8aaff5d3788c8a414c21286c71)