Description of problem:

Upgrading from 4.2.12 to 4.3.0-0.nightly-2019-12-20-145137 got stuck at 13%:

    Cluster did not complete upgrade: timed out waiting for the condition: Working towards 4.3.0-0.nightly-2019-12-20-145137: 13% complete

The mechine-config clusteroperator reported the following status conditions:

                    "conditions": [
                        {
                            "lastTransitionTime": "2019-12-20T16:16:38Z",
                            "message": "Cluster not available for 4.3.0-0.nightly-2019-12-20-145137",
                            "status": "False",
                            "type": "Available"
                        },
                        {
                            "lastTransitionTime": "2019-12-20T16:00:59Z",
                            "message": "Working towards 4.3.0-0.nightly-2019-12-20-145137",
                            "status": "True",
                            "type": "Progressing"
                        },
                        {
                            "lastTransitionTime": "2019-12-20T16:16:38Z",
                            "message": "Unable to apply 4.3.0-0.nightly-2019-12-20-145137: timed out waiting for the condition during syncRequiredMachineConfigPools: pool master has not progressed to latest configuration: controller version mismatch for rendered-master-f0cd2de7cae40c363de564c65600efa1 expected 23a6e6fb37e73501bc3216183ef5e6ebb15efc7a has d780d197a9c5848ba786982c0c4aaa7487297046, retrying",
                            "reason": "RequiredPoolsFailed",
                            "status": "True",
                            "type": "Degraded"
                        },
                        {
                            "lastTransitionTime": "2019-12-20T15:22:23Z",
                            "reason": "AsExpected",
                            "status": "True",
                            "type": "Upgradeable"
                        }
                    ],

Two of the nodes report "Kubelet stopped posting node status." in all their status conditions.

See https://prow.svc.ci.openshift.org/view/gcs/origin-ci-test/logs/release-openshift-origin-installer-e2e-aws-upgrade/13004

Additional info:

The "controller version mismatch" error looks similar to bug 1781141 ("controller version mismatch" when upgrading from 4.2.9 to 4.3).