stroage: Set default storage class

Setting a default storage class allows PersistentVolumes to be declared
without selecting a specific storage class in each object spec.

storage/longhorn.yaml

@@ -76,6 +76,8 @@ metadata:
     app.kubernetes.io/name: longhorn
     app.kubernetes.io/instance: longhorn
     app.kubernetes.io/version: v1.3.0
+  annotations:
+    storageclass.kubernetes.io/is-default-class: 'true'
 data:
   storageclass.yaml: |
     kind: StorageClass