[Filter] ?fields filter doesn't provide key attributes

[openBackhaul]

Resource paths shall be implemented like the RESTCONF interface at the SDN Controller NBI. At the RESTCONF interface, the ?fields filter result is automatically comprising the key attributes like uuid and local-id.

GET ... /core-model-1-4:network-control-domain=cache/control-construct=513559991A?fields=logical-termination-point%28ltp-augment-1-0%3Altp-augment-pac%3Blayer-protocol%28layer-protocol-name%29%29) does not list the key attributes.

[paulBMentopolis]

The internal structure of elements returned by MWDI is not modified by MDIP. The missing keys are already missing in the MWDI return payload.

[openBackhaul]

Issue has been initially addressed to the MDIP, but MDIP is passing through the response provided by MWDI.

[nardecchia-siae]

Tested with 1.1.2.d_impl