[cern] Labels override

Cherry-pick: https://review.opendev.org/#/c/720221/

The post for both clusters and nodegroups is adapted to wait for a boolean flag called merge_labels. Based on this flag the API will either merge the provided with the parent labels or just use the provided labels.

At the same time, the get methods of both clusters and nodegroups are adapted to include new fields in the response called "labels_overridden", "labels_added", "labels_skipped". The fields contain the differnces with the parent labels.

story: 2007515 task: 39691 Change-Id: I1054c54da96005a49e874de6f4cf60b5db57fc02