make storage class configurable

[eberlep]

Add config option to postgreslet
Use it when creating the individual postgres cluster

https://postgres-operator.readthedocs.io/en/latest/reference/cluster_manifest/#volume-properties

see https://github.com/metal-stack/csi-driver-lvm#installation , use csi-lvm-sc-mirror as default.

[majst01]

Chart to use is here:
https://github.com/metal-stack/helm-charts/tree/master/charts/csi-driver-lvm

[eberlep]

Already implemented in #172