Compute's DiskType.id field is never set

[DiskType.id](https://cloud.google.com/compute/docs/reference/latest/diskTypes#id) is documented but never returned by the service. See for instance the result of a GET:

``` json
{
 "kind": "compute#diskType",
 "creationTimestamp": "2014-10-17T16:09:45.266-07:00",
 "name": "local-ssd",
 "description": "Local SSD",
 "validDiskSize": "375GB-375GB",
 "zone": "https://content.googleapis.com/compute/v1/projects/gcloud-devel/zones/us-central1-a",
 "selfLink": "https://content.googleapis.com/compute/v1/projects/gcloud-devel/zones/us-central1-a/diskTypes/local-ssd",
 "defaultDiskSizeGb": "375"
}
```


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