Automatic merge from submit-queue

Instruct PLEG to detect pod sandbox state changes

This PR adds a Sandboxes list in `kubecontainer.Pod`, so that PLEG can check
sandbox changes using `GetPods()` . The sandboxes are treated as regular
containers (type `kubecontainer.Container`) for now to avoid additional
changes in PLEG.

/cc @feiskyer @yifan-gu @euank