[core] After copying a workflow using Hue 4 Copy button, saving the copied workflow fails

In Hue 4, user is able to copy the workflow, but unable to edit/save it.

The issue happens because we have document2 and document (old & obsolete in Hue 4) model objects and while copying the workflow, we actually don't create document object(as we don't need it anymore in Hue 4), and hence don't link document and document2 objects, but we have not updated the code when saving the workflow accordingly, that is we still try to save both document2 and document, https://github.com/cloudera/hue/blob/a7122cb0ed2072686cfb366571c87edc1f1044db/apps/oozie/src/oozie/models2.py#L3680.

For now we have fixed the issue by still linking the document objects on copy but in the future we need to clean up the code to actually remove document model objects.