Delete PVC /Volumes during down-scaling

[sdudoladov]

Right now Persistent volume claims (PVCs) are removed only on cluster deletion. In the case the number of pods in a PG cluster is simply reduced, PVCs/volumes of no longer running pods remain. We may consider removing them altogether.

[hjacobs]

It's good to see Postgres Operator handle cleaning up volumes in case of "scale down". kube-janitor can take care of deleting the PVCs when the StatefulSet is completely deleted: hjacobs/kube-janitor#62

[FxKu]

Now solved with persistent_volume_claim_retention_policyoption, see #2343