checkpointer: ignore Affinity within podspec (w/ tests)

[rphillips]

Kubernetes 1.12.x introduced new logic for Affinity [1]. In addition to
new logic, the Pod contains a default affinity. The new default affinity
gets serialized into the checkpoint file, and the 1.12.x kubelet does
not restore the pod due to the affinity.

This PR removes the affinity from the spec and documents that affinity's
are not supported.

"affinity": {
      "nodeAffinity": {
        "requiredDuringSchedulingIgnoredDuringExecution": {
          "nodeSelectorTerms": [
            {
              "matchExpressions": null
            }
          ]
        }
      }
    },

[1] kubernetes/kubernetes#68173
[2] https://github.com/kubernetes/kubernetes/blob/e39b510726113581c6f6a9c2db1753d794aa9cce/pkg/controller/daemon/util/daemonset_util.go#L183-L196

[k8s-ci-robot]

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