Adding extra data to cube portal_registry

* adding to registry
    * breakdown and country queries used on the indicators listing
      (scoreboard.visualization)
    * sparql endpoints used on the datacube and in the "Find breakdowns" and
      "Find countries" urls on the indicators listing
      (scoreboard.visualization)

* default registry values have been moved to variables in interfaces.defaults
  in order to read them if the registry has not been imported
* adding datacube_modifier in order to be able to read the default value
  from the registry