Fix the scripts to default empty string if EXTRA_DOCKER_OPTS isn't set.

cluster/gce/coreos/helper.sh

@@ -84,7 +84,7 @@ ADMISSION_CONTROL=$(yaml-quote ${ADMISSION_CONTROL:-})
 MASTER_IP_RANGE=$(yaml-quote ${MASTER_IP_RANGE})
 KUBERNETES_MASTER_NAME=$(yaml-quote ${MASTER_NAME})
 ZONE=$(yaml-quote ${ZONE})
-EXTRA_DOCKER_OPTS=$(yaml-quote ${EXTRA_DOCKER_OPTS})
+EXTRA_DOCKER_OPTS=$(yaml-quote ${EXTRA_DOCKER_OPTS:-})
 ENABLE_DOCKER_REGISTRY_CACHE=$(yaml-quote ${ENABLE_DOCKER_REGISTRY_CACHE:-false})
 PROJECT_ID=$(yaml-quote ${PROJECT})
 KUBERNETES_CONTAINER_RUNTIME=$(yaml-quote ${CONTAINER_RUNTIME})

cluster/gce/debian/helper.sh

@@ -70,7 +70,7 @@ EOF
     cat >>$file <<EOF
 KUBERNETES_MASTER: "false"
 ZONE: $(yaml-quote ${ZONE})
-EXTRA_DOCKER_OPTS: $(yaml-quote ${EXTRA_DOCKER_OPTS})
+EXTRA_DOCKER_OPTS: $(yaml-quote ${EXTRA_DOCKER_OPTS:-})
 ENABLE_DOCKER_REGISTRY_CACHE: $(yaml-quote ${ENABLE_DOCKER_REGISTRY_CACHE:-false})
 KUBELET_CERT: $(yaml-quote ${KUBELET_CERT_BASE64:-})
 KUBELET_KEY: $(yaml-quote ${KUBELET_KEY_BASE64:-})