Remove address from UpdateOpts

Address is immutable and should not be part of the `UpdateOpts` struct, per [the updateNode docs from Rackspace](http://docs.rackspace.com/loadbalancers/api/v1.0/clb-devguide/content/PUT_updateNode_v1.0__account__loadbalancers__loadBalancerId__nodes__nodeId__Nodes-d1e2173.html).

If you tried to change the IP previously, this error came back:

```
{"validationErrors":{"messages":["Node ip field cannot be modified."]},"message":"Validation Failure","code":400,"details":"The object is not valid"}
```

Now, well, you can't set the address.