Could use empty storage class as default

[solsson]

In #270 we switched to "standard" as the default storage class. Works out of the box with for example minikube and k3s. I had just guessed that it was some kind of convention, but I recently learned about https://kubernetes.io/docs/tasks/administer-cluster/change-default-storage-class/. Default volume claims could instead leave storage class unset. Done by for example the Minio helm chart.