Add Exec interface to VolumeHost

This exec should be used by volume plugins to execute mount utilities.
It will eventually execute things in mount containers.

pkg/kubelet/volume_host.go

@@ -156,3 +156,7 @@ func (kvh *kubeletVolumeHost) GetNodeLabels() (map[string]string, error) {
 	}
 	return node.Labels, nil
 }
+
+func (kvh *kubeletVolumeHost) GetExec(pluginName string) mount.Exec {
+	return mount.NewOsExec()
+}