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/util/mount/mount.go

@@ -70,6 +70,16 @@ type Interface interface {
 	GetDeviceNameFromMount(mountPath, pluginDir string) (string, error)
 }
 
+// Exec executes command where mount utilities are. This can be either the host,
+// container where kubelet runs or even a remote pod with mount utilities.
+// Usual pkg/util/exec interface is not used because kubelet.RunInContainer does
+// not provide stdin/stdout/stderr streams.
+type Exec interface {
+	// Run executes a command and returns its stdout + stderr combined in one
+	// stream.
+	Run(cmd string, args ...string) ([]byte, error)
+}
+
 // Compile-time check to ensure all Mounter implementations satisfy
 // the mount interface
 var _ Interface = &Mounter{}