MDSAL-17: Binding V2 - Simplify InstanceIdentifer creation

MDSAL-17: Binding V2 - Simplify InstanceIdentifer creation

- Add a static method getInstanceIdentifier to generated interface
  of datastore nodes:

  - If there are list items, just toke its key as inputs, like:
    getInstanceIdentifier(key1, key2,...),

  -Or it would simply be getInstanceIdentifier().

- For choice/case's children data nodes, II does not reference
  choice or case but through them.

- Add unit tests.

Change-Id: Iea592557668fd7a8ab8e6874caaac7b9e834a859
Signed-off-by: Jie Han <han.jie@zte.com.cn>