Add optional cluster_domain_suffix variable

* Allow kube-dns to respond to DNS queries with a custom
suffix, instead of the default 'cluster.local'
* Useful when multiple clusters exist on the same local
network and wish to query services on one another

google-cloud/container-linux/kubernetes/controllers/cl/controller.yaml.tmpl

@@ -74,7 +74,7 @@ systemd:
           --anonymous-auth=false \
           --client-ca-file=/etc/kubernetes/ca.crt \
           --cluster_dns=${k8s_dns_service_ip} \
-          --cluster_domain=cluster.local \
+          --cluster_domain=${cluster_domain_suffix} \
           --cni-conf-dir=/etc/kubernetes/cni/net.d \
           --exit-on-lock-contention \
           --kubeconfig=/etc/kubernetes/kubeconfig \