Reinstate crd:preserveUnknownFields option removed by kubernetes-sigs#607
to allow specifying the value of deprecated spec.preserveUnknownFields
CRD field.

This is useful for updating CRDs that were automatically converted from
v1beta1 version which had true as a default value and resulted in:
```
$ kubectl get crd foo.bar -o yaml | grep preserveUnknownFields
  preserveUnknownFields: true
    message: 'spec.preserveUnknownFields: Invalid value: true: must be false'
```

For kubernetes-sigs#476

…elds