Automatic merge from submit-queue

rkt: Refactor GarbageCollect to enforce GCPolicy.

Previously, we uses `rkt gc` to garbage collect dead pods, which is very coarse, and can cause the dead pods to be removed too aggressively. 

This PR improves the garbage collection, now after one GC iteration:
- The deleted pods will be removed.
- If the number of containers exceeds gcPolicy.MaxContainers,
  then containers whose ages are older than gcPolicy.minAge will be removed.

cc @kubernetes/sig-node @euank @sjpotter 

Pending on #23887 for the Godep updates.

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