Add ErrorDocument for docs.o.o

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

modules/openstack_project/templates/docs.vhost.erb

@@ -20,6 +20,7 @@ NameVirtualHost <%= @vhost_name %>:<%= @port %>
         Require all granted
         # Allow mod_rewrite rules
         AllowOverride FileInfo
+        ErrorDocument 404 /errorpage.html
     </Directory>
 
     ErrorLog /var/log/<%= scope.lookupvar("httpd::params::apache_name") %>/<%= @name %>_error.log