Fixes#5857
### Motivation
With current aproach for specifying storage class in persistent volume claim it's not possible to customize the provisioner parameters. If the property 'storageClass' is declared the chart always create a new storage class with hardcoded parameters.
### Modifications
A property 'storageClassName' was added to support an existent storage class.
### Verifying this change
This change is a trivial rework / code cleanup without any test coverage.
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