fix: use default storage class when class is not defined

[MasayaAoyama]

Thank you for your great works! I fixed some behavior.

This PR is for v4 contribution of #430 .

Description

use default storage class when class is not defined

Relevant issues/tickets

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• This change requires a documentation update
• I have added tests that prove my fix is effective or that my feature works
• I have added a test case that will be used to verify my changes
• Verified independently on a cluster by reviewer

Verification steps

When Grafana.spec.dataStorage.class is not defined or set nil, generated PVC.spec.storageClassName is set "".
But I think we prefer to use default storageClass.

apiVersion: integreatly.org/v1alpha1
kind: Grafana
metadata:
  name: grafana
spec:
  baseImage: grafana/grafana:7.4.3
  ...
  dataStorage:
    accessModes:
    - ReadWriteOnce
    size: 5Gi
#    class: null

• default bahavior

$ k -n grafana get pvc grafana-pvc -oyaml | k neat
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: grafana-pvc
  namespace: grafana
spec:
  accessModes:
  - ReadWriteOnce
  resources:
    requests:
      storage: 5Gi
  storageClassName: ""

• newer bahavior

$ k -n grafana get pvc grafana-pvc -oyaml | k neat
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: grafana-pvc
  namespace: grafana
spec:
  accessModes:
  - ReadWriteOnce
  resources:
    requests:
      storage: 5Gi
  storageClassName: my-default-storageclass

[MasayaAoyama]

Cc: @pb82 @HubertStefanski