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/plugins.go

@@ -244,6 +244,9 @@ type VolumeHost interface {
 	// Returns a function that returns a configmap.
 	GetConfigMapFunc() func(namespace, name string) (*v1.ConfigMap, error)
 
+	// Returns an interface that should be used to execute any utilities in volume plugins
+	GetExec(pluginName string) mount.Exec
+
 	// Returns the labels on the node
 	GetNodeLabels() (map[string]string, error)
 }