Change bootstrap host to replace itself with machine-os-content too

Currently every machine instance we launch uses the same "bootimage", 
including the bootstrap host. However, everything except bootstrap (i.e. 
control plane and workers) replace their OS content with the 
machine-os-content from the release payload before joining the cluster.

For more information, see: 
https://github.com/openshift/machine-config-operator/blob/master/docs/OSUpgrades.md

This changes the bootstrap host to do the same, which will help avoid 
issues from "bootimage drift".

Closes: #2542

data/data/bootstrap/files/usr/local/bin/bootkube.sh.template

@@ -37,6 +37,21 @@ MDNS_PUBLISHER_IMAGE=$(image_for mdns-publisher)
 HAPROXY_IMAGE=$(image_for haproxy-router)
 BAREMETAL_RUNTIMECFG_IMAGE=$(image_for baremetal-runtimecfg)
 
+# Copy machine-config-daemon binary from payload and run pivot
+if [ ! -f .pivot-done ]; then
+  mkdir /run/pivot /etc/pivot
+  touch /run/pivot/reboot-needed
+  echo "${MACHINE_CONFIG_OSCONTENT}" > /etc/pivot/image-pullspec
+
+  hostmcd=/usr/local/bin/machine-config-daemon
+  bootkube_podman_run --entrypoint=sh "${MACHINE_CONFIG_OPERATOR_IMAGE}" \
+    cat /usr/bin/machine-config-daemon > ${hostmcd}
+  chmod a+x ${hostmcd}
+  restorecon ${hostmcd}
+  touch .pivot-done
+  ${hostmcd} pivot
+fi
+
 mkdir --parents ./{bootstrap-manifests,manifests}
 
 if [ ! -f openshift-manifests.done ]