Use example settings in horizon repo as local_settings.py

The current horizon_settings.py in devstack is out-of-date and we
tend to forget to update this file. This commit changes devstack
to use the example settings in horizon repo.

Change-Id: I0bb6af21a806a72ed59f31b094dd21da85ca335e
Merge remote-tracking branch 'm_src_repo/citrix-fixes' into build-temp