Add KUBELET_IMAGE to Fedora CoreOS bootstrap.service (#945)

* Align with Flatcar Linux `bootstrap.service`

bare-metal/fedora-coreos/kubernetes/fcc/controller.yaml

@@ -120,6 +120,7 @@ systemd:
         Type=oneshot
         RemainAfterExit=true
         WorkingDirectory=/opt/bootstrap
+        Environment=KUBELET_IMAGE=quay.io/poseidon/kubelet:v1.20.4
         ExecStartPre=-/usr/bin/podman rm bootstrap
         ExecStart=/usr/bin/podman run --name bootstrap \
             --network host \
@@ -127,7 +128,7 @@ systemd:
             --volume /opt/bootstrap/assets:/assets:ro,Z \
             --volume /opt/bootstrap/apply:/apply:ro,Z \
             --entrypoint=/apply \
-            quay.io/poseidon/kubelet:v1.20.4
+            $${KUBELET_IMAGE}
         ExecStartPost=/bin/touch /opt/bootstrap/bootstrap.done
         ExecStartPost=-/usr/bin/podman stop bootstrap
 storage: