At present, when a cluster is patched without node_count (e.g. when
health_status and health_status_reason are patched), the node_count in
the cluster_update invocation contains cluster.node_count instead of
default_ng_worker.node_count which causes unexpected behaviour when a
cluster has additional nodegroups (e.g. the default-worker size gets
incremented by 1 if the additional nodegroup has 1 worker each time the
cluster is patched).
This PS fixes this issue by using default_ng_worker.node_count if path
"/node_count" is not present in the patch object.
(cherry picked from commit 55fd12a47caba23db96a34a8a8ee603cae12f841)