userdata: Set kubelet config path

The default configuration for the *kubelet.service* unit does not
specify the path to the `config.yml` generated by `kubeadm`.  Thus, any
settings defined in the `kublet-config` ConfigMap do not take effect.
To resolve this, we have to explicitly set the path in the `config`
property of the `kubeletExtraArgs` object in the join configuration.

terraform/userdata.yml

@@ -69,6 +69,7 @@ write_files:
       kubeletExtraArgs:
         provider-id: aws:///${az}/${instance_id}
         node-ip: ${internal_ip}
+        config: /var/lib/kubelet/config.yaml
     discovery:
       file:
         kubeConfigPath: ${BASE_URL}/kubeadm/kubeconfig/${instance_id}