modify chart's yaml so that user can use default storage class

[jeremyxu2010]

Refer to storageclass's usage specification, modify the chart so that user can use default storageclass.

redis-ha-statefulset.yaml

    {{- if .Values.persistentVolume.storageClass }}
    {{- if (eq "-" .Values.persistentVolume.storageClass) }}
      storageClassName: ""
    {{- else }}
      storageClassName: "{{ .Values.persistentVolume.storageClass }}"
    {{- end }}
    {{- end }}

values.yaml

  ## If defined, storageClassName: <storageClass>
  ## If set to "-", storageClassName: "", which disables dynamic provisioning
  ## If undefined (the default) or set to null, no storageClassName spec is
  ##   set, choosing the default provisioner.  (gp2 on AWS, standard on
  ##   GKE, AWS & OpenStack)
  ##
  # storageClass: "-"

[lwolf]

thanks for the PR.