Add additional checks before PVC deletion

[burmanm]

What is missing?

While Kubernetes has pvc-protection that checks if PersistentVolumeClaim is still used by a pod, we could do the same before issuing pvc deletion.

Why is this needed?

In some cases (such as migrations / error cases / restore failures), users might have had orphaned pods + PVCs and the generated CassandraDatacenter wasn't correct. Deleting this CassandraDatacenter might have deleted the PVCs also as their name pair (clusterName + datacenterName) will match the PVCs, even if the Pods themselves are not actually part of the datacenter.

Thus, lets prevent this from happening and prevent the PVC deletion if a PVC is actually still in use.

┆Issue is synchronized with this Jira Story by Unito
┆Issue Number: CASS-82