BUG-1344: Implemented getOriginal() for DataSchemaNode.

For nodes which were added by uses method getOriginal returns original node definition from grouping.
If node wasn't added by uses returns null.
Added tests.

Change-Id: Ib75ac2d9dbbd69d68a8e99333fe005f095fce90a
Signed-off-by: Martin Vitez <mvitez@cisco.com>
Merge "BUG-1344: Implemented getOriginal() for DataSchemaNode."