Adapt hc2vpp to ODL Oxygen (HONEYCOMB-424)

Adapt hc2vpp to ODL Oxygen (HONEYCOMB-424)

Requires https://gerrit.fd.io/r/#/c/11320/

Also changes encoding of empty type in json test files
to [null] (RFC8040) from {} that was used so far.

Honeycomb uses draft-ietf-netconf-restconf-02
of restconf compoments, so there might be
a bug in ODL or HC configuration.

To resolve it,
using RFC8040 version of ODL restconf components
should be considered (HONEYCOMB-430).

Not covered:
- postman collections update
- CSIT tests update

Change-Id: I820c46b5b0b22330e8529e8870c8bdad9508473b
Signed-off-by: Marek Gradzki <[email protected]>