Add file to hold nodes on iptables-legacy

* Add `/etc/fedora-coreos/iptables-legacy.stamp` to declare
that `iptables-legacy` should be used instead of `iptables-nft`
(until support is added in future releases)
* https://github.com/coreos/fedora-coreos-tracker/issues/676

google-cloud/fedora-coreos/kubernetes/fcc/controller.yaml

@@ -212,6 +212,7 @@ storage:
           ETCD_PEER_CERT_FILE=/etc/ssl/certs/etcd/peer.crt
           ETCD_PEER_KEY_FILE=/etc/ssl/certs/etcd/peer.key
           ETCD_PEER_CLIENT_CERT_AUTH=true
+    - path: /etc/fedora-coreos/iptables-legacy.stamp
 passwd:
   users:
     - name: core

google-cloud/fedora-coreos/kubernetes/workers/fcc/worker.yaml

@@ -124,6 +124,7 @@ storage:
           DefaultCPUAccounting=yes
           DefaultMemoryAccounting=yes
           DefaultBlockIOAccounting=yes
+    - path: /etc/fedora-coreos/iptables-legacy.stamp
 passwd:
   users:
     - name: core