Fixes #5107: Enable ncf builder in Rudder vhost

Ticket: http://www.rudder-project.org/redmine/issues/5107

To be merged in 2.11
Why is this block of configuration not in  /opt/rudder/etc/rudder-apache-common.conf ? It seems this would be a much better place to have it, rather than right in this file. Or am I missing something?

But actually, either way, I would like this apache config snippet to be included with the ncf-api-virtualenv package (it is very specific to that package anyway, so it should live with it), and be included from there.