get_node_utilization_json API needs to work correctly across CPU-only builds

[tpatki]

The API works correctly on GPU-only and multi-arch build, but errors out with gpu_json unimplemented error for CPU-only build. It should be skipping the GPU part and only reporting CPU/memory utilization for correct behavior.

Also, rename this from get_node_utilization_json to get_utilization_json, similar to other JSON APIs.