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/volume/testing/testing.go

@@ -49,6 +49,7 @@ type fakeVolumeHost struct {
 	pluginMgr  VolumePluginMgr
 	cloud      cloudprovider.Interface
 	mounter    mount.Interface
+	exec       mount.Exec
 	writer     io.Writer
 }
 
@@ -64,6 +65,7 @@ func newFakeVolumeHost(rootDir string, kubeClient clientset.Interface, plugins [
 	host := &fakeVolumeHost{rootDir: rootDir, kubeClient: kubeClient, cloud: cloud}
 	host.mounter = &mount.FakeMounter{}
 	host.writer = &io.StdWriter{}
+	host.exec = mount.NewFakeExec(nil)
 	host.pluginMgr.InitPlugins(plugins, host)
 	return host
 }
@@ -142,6 +144,10 @@ func (f *fakeVolumeHost) GetSecretFunc() func(namespace, name string) (*v1.Secre
 	}
 }
 
+func (f *fakeVolumeHost) GetExec(pluginName string) mount.Exec {
+	return f.exec
+}
+
 func (f *fakeVolumeHost) GetConfigMapFunc() func(namespace, name string) (*v1.ConfigMap, error) {
 	return func(namespace, name string) (*v1.ConfigMap, error) {
 		return f.kubeClient.Core().ConfigMaps(namespace).Get(name, metav1.GetOptions{})