Fixed mounting with containerized kubelet.

Kubelet was not able to mount volumes when running inside a container and
using nsenter mounter,

NsenterMounter.IsLikelyNotMountPoint() should return ErrNotExist when the
checked directory does not exists as the regular mounted does this and
some volume plugins depend on this behavior.

pkg/util/mount/nsenter_mount.go

@@ -170,6 +170,12 @@ func (n *NsenterMounter) IsLikelyNotMountPoint(file string) (bool, error) {
 		return true, err
 	}
 
+	// Check the directory exists
+	if _, err = os.Stat(file); os.IsNotExist(err) {
+		glog.V(5).Infof("findmnt: directory %s does not exist", file)
+		return true, err
+	}
+
 	args := []string{"--mount=/rootfs/proc/1/ns/mnt", "--", n.absHostPath("findmnt"), "-o", "target", "--noheadings", "--target", file}
 	glog.V(5).Infof("findmnt command: %v %v", nsenterPath, args)