kolla-ansible should provide a mechanism to allow operators to overwrite
the default (or not exposed through configuration) options for the
local_settings.
local_settings.j2 may be good place to configure horizon but requires
operator to sync it every release.
custom_local_settings.j2 can be used to overwrite things from
local_settings.j2 without a need of syncing it first.
This patch also adds a release note and a documentation section
under the advanced configuration page.
Partial-Bug: #1769970
Depends-On: https://review.openstack.org/#/c/567006/
Change-Id: I84b54ba737276114e512d4577ac4b9010682bb98