Validate partial document

This DaemonSet is incomplete yet kubeval validates as ok.

```
[jodewey:~/git/konfig] 2 % kubeval foo.yaml
The document foo.yaml contains a valid DaemonSet
[jodewey:~/git/konfig] % kubectl apply --dry-run --validate -f foo.yaml
daemonset.apps "foo" created (dry run)
[jodewey:~/git/konfig] % cat foo.yaml
apiVersion: apps/v1
kind: DaemonSet
metadata:
name: foo
namespace: foo
```

I would expect kubeval to report an issue, esp when not fully complete. I understand the data is valid for an update but not an initial deployment.