Add DeviceConfiguration NetJSON support

[arun1993]

As the JSON plugin in SCAL can't provide support for NetJSON schema, we can extend the SCAL APIs to any object in the form of JSON.

1. Publish NetJSON schema as different objects on the SCAL to do this. This is done as shown in the examples/json directory.
2. Or For those data-models which don't have support of the JSON plugin, we can create a ubus object and make it available at the JSON plugin by adding a new API to call and filter the ubus object.

[arun1993]

This issue is targeted to finish completely before 2nd evaluation. However one part would be finished for the 1st evaluation phase itself. Issue will be created for that when submitting the patch.

[nbd168]

I think the JSON plugin should not emit a full JSON object. The existing abstraction of objects and parameters should be used instead, and you could make the netjson CLI (or library) assemble the NetJSON structure from the data that SCAL provides.
We could look into extending the API to allow SCAL to provide multiple parameters at once to speed up the process, but for the initial prototype it probably makes sense to stick with the current SCAL API.