We now have an errordocument, enable using it for 404 errors.
The content is served from openstack-manuals.
Alternative would be to use .htaccess but then the page would be
available on both old and new pages - and the content is specific to the
new AFS based page. Therefore, I choose to use ErrorDocument here.
Change-Id: Iefa7c4453f05071dd990a5ae8a29d2648bb797d9