set default k8s version during make cluster

fixed #97

Author: Gianluca Arbezzano

scripts/generate-cluster.sh

@@ -54,7 +54,7 @@ SERVICE_CIDR=${SERVICE_CIDR:-${DEFAULT_SERVICE_CIDR}}
 WORKER_NODE_TYPE=${WORKER_NODE_TYPE:-${DEFAULT_WORKER_NODE_TYPE}}
 MASTER_NODE_TYPE=${MASTER_NODE_TYPE:-${DEFAULT_MASTER_NODE_TYPE}}
 NODE_OS=${NODE_OS:-${DEFAULT_NODE_OS}}
-KUBERNETES_VERSION=${KUBERNETES_VERSION:${DEFAULT_KUBERNETES_VERSION}}
+KUBERNETES_VERSION=${KUBERNETES_VERSION:-${DEFAULT_KUBERNETES_VERSION}}
 SSH_KEY=${SSH_KEY:-""}
 
 PROJECT_ID=${PACKET_PROJECT_ID}