feature: support to use the default StorageClass

[veezhang]

Fix: #34

[MegaByte875]

I think here is check null pointer to avoid panic, the emptly value is legal, right? If use default storageClass, the filed can remove while scName is nil.

[veezhang]

It's legal but it doesn't seem to make much sense, so I set it to nil when empty.
Are there any other problems that I haven't found?

Here is a example:

$ kubectl get sc
NAME                 PROVISIONER             RECLAIMPOLICY   VOLUMEBINDINGMODE      ALLOWVOLUMEEXPANSION   AGE
standard (default)   rancher.io/local-path   Delete          WaitForFirstConsumer   false                  11h

$ cat ./yaml/storageclass-test.yaml
apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: test
spec:
  serviceName: test
  selector:
    matchLabels:
      app: test
  replicas: 1
  template:
    metadata:
      labels:
        app: test
    spec:
      containers:
        - name: test
          image: vesoft/nebula-console:v2-nightly
          command:
            - sleep
            - "10000"
          ports:
            - containerPort: 80
              name: test
          volumeMounts:
            - name: disk
              mountPath: /data
  volumeClaimTemplates:
    - metadata:
        name: disk
      spec:
        accessModes: [ "ReadWriteOnce" ]
        storageClassName: ""
        resources:
          requests:
            storage: 20Gi
$ kubectl apply -f ./yaml/storageclass-test.yaml
$ kubectl describe pvc disk-test-0
Name:          disk-test-0
Namespace:     default
StorageClass:
Status:        Pending
Volume:
Labels:        app=test
Annotations:   <none>
Finalizers:    [kubernetes.io/pvc-protection]
Capacity:
Access Modes:
VolumeMode:    Filesystem
Used By:       test-0
Events:
  Type    Reason         Age               From                         Message
  ----    ------         ----              ----                         -------
  Normal  FailedBinding  7s (x4 over 42s)  persistentvolume-controller  no persistent volumes available for this claim and no storage class is set