kubeadm: change os.Remove to os.RemoveAll

This would not correctly clean out the tmp dir, since os.Remove doesn't
recursively remove dirs, which could exist in the tmp dir.

cmd/kubeadm/app/phases/certs/certs_test.go

@@ -32,7 +32,7 @@ func TestCreatePKIAssets(t *testing.T) {
 	if err != nil {
 		t.Fatalf("Couldn't create tmpdir")
 	}
-	defer os.Remove(tmpdir)
+	defer os.RemoveAll(tmpdir)
 
 	var tests = []struct {
 		cfg      *kubeadmapi.MasterConfiguration