Reuse k8s drain code

This depends on kubernetes/kubernetes#42115

cmd/kops/rollingupdatecluster.go

@@ -298,6 +298,7 @@ func RunRollingUpdateCluster(f *util.Factory, out io.Writer, options *RollingUpd
 		Force:            options.Force,
 		Cloud:            cloud,
 		K8sClient:        k8sClient,
+		ClientConfig:        kutil.NewClientConfig(config, "kube-system"),
 		FailOnDrainError: options.FailOnDrainError,
 		FailOnValidate:   options.FailOnValidate,
 		CloudOnly:        options.CloudOnly,

upup/pkg/kutil/config.go

@@ -0,0 +1,43 @@
+package kutil
+
+import (
+	"k8s.io/kubernetes/pkg/client/restclient"
+	"fmt"
+	"k8s.io/kubernetes/pkg/client/unversioned/clientcmd"
+	"github.com/golang/glog"
+	clientcmdapi "k8s.io/kubernetes/pkg/client/unversioned/clientcmd/api"
+)
+
+func NewClientConfig(clientConfig *restclient.Config, namespace string) clientcmd.ClientConfig {
+	c := &SimpleClientConfig{clientConfig: clientConfig, namespace: namespace}
+	return c
+}
+
+// ClientConfig is used to make it easy to get an api server client
+type SimpleClientConfig struct {
+	clientConfig *restclient.Config
+	namespace string
+}
+
+var _ clientcmd.ClientConfig = &SimpleClientConfig{}
+
+// RawConfig returns the merged result of all overrides
+func (c*SimpleClientConfig) RawConfig() (clientcmdapi.Config, error) {
+	return clientcmdapi.Config{}, fmt.Errorf("SimpleClientConfig::RawConfig not implemented")
+}
+
+// ClientConfig returns a complete client config
+func (c*SimpleClientConfig) ClientConfig() (*restclient.Config, error) {
+	return c.clientConfig, nil
+}
+// Namespace returns the namespace resulting from the merged
+// result of all overrides and a boolean indicating if it was
+// overridden
+func (c*SimpleClientConfig) Namespace() (string, bool, error) {
+	return c.namespace, false, nil
+}
+// ConfigAccess returns the rules for loading/persisting the config.
+func (c*SimpleClientConfig) ConfigAccess() clientcmd.ConfigAccess {
+	glog.Fatalf("SimpleClientConfig::RawConfig not implemented")
+	return nil
+}