Fixes #5107: Enable ncf builder in Rudder vhost

Ticket: http://www.rudder-project.org/redmine/issues/5107

To be merged in 2.11
In the same spirit as my comment on https://github.com/Normation/rudder-packages/pull/395 and my comment below, I think this module activation should happen in the ncf-api-virtualenv package's postinst, instead.