Automatic merge from submit-queue

Support for storage class for vSphere volume plugin. Custom disk format for dynamic provisioning.

This PR does following,

1.  Add support for storage class for vSphere volume plugin.
2. Add option for user to provision disk with different disk formats. Format choices are
    "thin" (default), "zeroedthick", "eagerzeroedthick".

Sample storageclass (yaml):
```
kind: StorageClass
apiVersion: storage.k8s.io/v1beta1
metadata:
  name: slow
provisioner: kubernetes.io/vsphere-volume
parameters:
  diskformat: thin
```