Group container-runtime-specific flags/options together

Do not store them in kubelet's configuration. Eventually, we would like
to deprecate all these flags as they should not be part of kubelet.

cmd/kubelet/kubelet.go

@@ -46,7 +46,7 @@ func main() {
 	verflag.PrintAndExitIfRequested()
 
 	if s.ExperimentalDockershim {
-		if err := app.RunDockershim(&s.KubeletConfiguration, s.DockershimRootDirectory); err != nil {
+		if err := app.RunDockershim(&s.KubeletConfiguration, &s.ContainerRuntimeOptions); err != nil {
 			fmt.Fprintf(os.Stderr, "error: %v\n", err)
 			os.Exit(1)
 		}