This changes the existing cluster APIs and the cluster conductor to
take into consideration nodegroups:
* create: now creates the default nodegroups for the cluster
* update: updates the default nodegroups of the cluster
* delete: deletes also the nodegroups that belong to the cluster
* cluster_resize: takes into account the nodegroup provided by the API
story: 2005266
Change-Id: I5478c83ca316f8f09625607d5ae9d9f3c02eb65a