Added update ResourceClass

To add or remove Racks on a resource class simply do an update and alter the racks array attribute accordingly.

```
  curl -iX PUT -H 'Content-Type: application/json' -H 'Accept: application/json' -d '
  {
    "service_type": "compute",
    "name": "test-chassis",
    "racks":[
      {
        "id": 1,
        "links": [
          { 
            "href":"http://0.0.0.0:6385/v1/racks/1",
            "rel":"self"
          }
        ]
      }
    ]
  }
' http://0.0.0.0:6385/v1/resource_classes/13
```