Handle UNMODIFIED type in DataTreeCandidateInputOutput

We now replicate DataTreeCandidates with root node type UNMODIFIED
however the deserialization in DataTreeCandidateInputOutput throws
an ISE with "Unhandled node type 2". The writeDataTreeCandidate
method writes the UNMODIFIED type but readDataTreeCandidate needs
to handle that case as well.

In addition, writeDataTreeCandidate also handles APPEARED and
DISAPPEARED but readDataTreeCandidate does not so I made that
change as well.

Change-Id: I19932e545bbeb95fa10d5f57d43a5780af586285
Signed-off-by: Tom Pantelis <tpanteli@brocade.com>