rkt: Add post-start hook support.

This adds a poll-and-timeout procedure after the pod is
started, to make sure the post-start hooks execute when the
container is actually running.

This is a temporal workaround for implementing post-hooks,
a long term solution is to use lifecycle event to trigger
those hooks, see https://github.com/kubernetes/kubernetes/issues/23084.

Also this fixes a bug of getting container ID for a non-running
container when running pre-stop hook.

pkg/kubelet/rkt/container_id.go

@@ -46,7 +46,7 @@ func buildContainerID(c *containerID) kubecontainer.ContainerID {
 func parseContainerID(id kubecontainer.ContainerID) (*containerID, error) {
 	tuples := strings.Split(id.ID, ":")
 	if len(tuples) != 2 {
-		return nil, fmt.Errorf("rkt: cannot parse container ID for: %v", id)
+		return nil, fmt.Errorf("rkt: cannot parse container ID for: %q, required format is [UUID:APPNAME]", id)
 	}
 	return &containerID{
 		uuid:    tuples[0],