don't export draft children of orphaned drafted (TNL-923)

@dmitchell 
@zubair-arbi 

Fixes an issue where the draft children of orphaned drafts were being exported

related bugs:
https://openedx.atlassian.net/browse/TNL-923
https://openedx.atlassian.net/browse/TNL-872
Instead of ignoring orphans here can we just ignore them while making "draft_node_list" at line#137 ?