Refactor pkg/util/mount to be more reusable

This patch refactors pkg/util/mount to be more usable outside of
Kubernetes. This is done by refactoring mount.Interface to only contain
methods that are not K8s specific. Methods that are not relevant to
basic mount activities but still have OS-specific implementations are
now found in a mount.HostUtils interface.

cmd/kubelet/app/server.go

@@ -372,6 +372,7 @@ func UnsecuredDependencies(s *options.KubeletServer) (*kubelet.Dependencies, err
 
 	mounter := mount.New(s.ExperimentalMounterPath)
 	subpather := subpath.New(mounter)
+	hu := mount.NewHostUtil()
 	var pluginRunner = exec.New()
 	if s.Containerized {
 		klog.V(2).Info("Running kubelet in containerized mode")
@@ -382,6 +383,7 @@ func UnsecuredDependencies(s *options.KubeletServer) (*kubelet.Dependencies, err
 		mounter = nsutil.NewMounter(s.RootDirectory, ne)
 		// NSenter only valid on Linux
 		subpather = subpath.NewNSEnter(mounter, ne, s.RootDirectory)
+		hu = nsutil.NewHostUtil(ne, s.RootDirectory)
 		// an exec interface which can use nsenter for flex plugin calls
 		pluginRunner, err = nsenter.NewNsenter(nsenter.DefaultHostRootFsPath, exec.New())
 		if err != nil {
@@ -407,6 +409,7 @@ func UnsecuredDependencies(s *options.KubeletServer) (*kubelet.Dependencies, err
 		KubeClient:          nil,
 		HeartbeatClient:     nil,
 		EventClient:         nil,
+		HostUtil:            hu,
 		Mounter:             mounter,
 		Subpather:           subpather,
 		OOMAdjuster:         oom.NewOOMAdjuster(),