Automatic merge from submit-queue

Don't require failureDomains in PodAffinityChecker

`failureDomains` are only used for `PreferredDuringScheduling` pod
anti-affinity, which is ignored by `PodAffinityChecker`.
This unnecessary requirement was making it hard to move
`PodAffinityChecker` to `GeneralPredicates` because that would require
passing `--failure-domains` to both `kubelet` and `kube-controller-manager`.