Need to add clarity around orchestrator's copy behavior for script

Per discussion on 05/07/15 TOSCA TC Special 4 Hour Meeting:

(Luc) :
Default behavior: Assumed that orchestrators execute relative to the CSAR and removes on completion.

Now that we have get_artifact() function which allows template author to control placement and persistence of an artifact if they want to control it directly.

Luc: Perhaps we do not want to copy on every compute node if for example we have 5 compute nodes.

Note: orchestrators should only copy artifacts for that node to that compute not all artifacts to all nodes.
(keep wordpress scripts on its compute and mysql on its compute).

Needs to reference the 'hosting' (or containment) hirarachy.