Expose Horizon "DocumentRoot" on host

This will allow webroot plugin for Let's Encrypt to actually work.
The container has no need to write in this location.

Change-Id: Ia76a0cc007abfdec6f25e1371eb696864f2925fd
Closes-Bug: 1768519

docker/services/horizon.yaml

@@ -132,6 +132,7 @@ outputs:
                   - /var/lib/config-data/puppet-generated/horizon/:/var/lib/kolla/config_files/src:ro
                   - /var/log/containers/horizon:/var/log/horizon
                   - /var/log/containers/httpd/horizon:/var/log/httpd
+                  - /var/www/:/var/www/:ro
                   -
                     if:
                       - internal_tls_enabled