message: add managed by in snapshot
diff: },
  "Snapshot": {
  "properties": {
  + "managedBy": {
  + "readOnly": true,
  + "type": "string",
  + "description": "A relative URI containing the ID of the VM that has the disk attached."
  + },
  "sku": {
  "$ref": "#/definitions/DiskSku"
  },
  