Rework method of updating atomic-updated data volumes

This change affects the way that secret, configmap, downwardAPI and projected
volumes (which all use the same underlying code) implement their data update
functionality.

* Instead of creating a subdirectory hierarchy that itself
  will contain symlinks to each actual data file, create only
  symlinks to items in the root of the volume, whether they
  be files or directories.
* Rather than comparing the user-visible data directory
  to see if an update is needed, compare with the current
  version of the data directory.
* Fix data dir timestamp format year
* Create ..data symlink even when a data volume has no data so
  consumers can have simplified update watch logic.

pkg/volume/configmap/configmap_test.go

@@ -466,13 +466,35 @@ func TestPluginOptional(t *testing.T) {
 		}
 	}
 
+	datadirSymlink := path.Join(volumePath, "..data")
+	datadir, err := os.Readlink(datadirSymlink)
+	if err != nil && os.IsNotExist(err) {
+		t.Fatalf("couldn't find volume path's data dir, %s", datadirSymlink)
+	} else if err != nil {
+		t.Fatalf("couldn't read symlink, %s", datadirSymlink)
+	}
+	datadirPath := path.Join(volumePath, datadir)
+
 	infos, err := ioutil.ReadDir(volumePath)
 	if err != nil {
 		t.Fatalf("couldn't find volume path, %s", volumePath)
 	}
 	if len(infos) != 0 {
-		t.Errorf("empty directory, %s, not found", volumePath)
+		for _, fi := range infos {
+			if fi.Name() != "..data" && fi.Name() != datadir {
+				t.Errorf("empty data directory, %s, is not empty. Contains: %s", datadirSymlink, fi.Name())
+			}
+		}
 	}
+
+	infos, err = ioutil.ReadDir(datadirPath)
+	if err != nil {
+		t.Fatalf("couldn't find volume data path, %s", datadirPath)
+	}
+	if len(infos) != 0 {
+		t.Errorf("empty data directory, %s, is not empty. Contains: %s", datadirSymlink, infos[0].Name())
+	}
+
 	doTestCleanAndTeardown(plugin, testPodUID, testVolumeName, volumePath, t)
 }