This adds a boolean flag called --merge-labels at the create CLIs
for clusters and nodegroups. The show output for clusters and
nodegroups is modified to contain also new fields called
"labels_overridden", "labels_added", "labels_skipped". The new labels
contain the differences of the object's labels with its parent's.
story: 2007515
task: 39692
Depends-On: I1054c54da96005a49e874de6f4cf60b5db57fc02
Change-Id: Ie68bb85f4b8bbacc5b71b523d8639cf30e6e1992
0febfc728f