Add storageclass to the root values.yaml file

Current values.yaml doesn't explicitly mentions how to use a custom storageClass for etcd. I guess either set a global one or add a etcd.persistence.storageClassName section on the root values.yaml file.

Read the root values.yaml first and trace the existing etcd persistence configuration to determine whether a global storage class or etcd.persistence.storageClassName matches the chart's conventions. Done means the chosen storageClass option is explicitly exposed there and applies to the intended etcd configuration; run the repository's available Helm validation if present.

Written by the indexing model from the issue text.