flanneld: SubnetLen 21 -> 24; fix #576

Author: Zhang Jun

05.部署flannel网络.md

@@ -115,7 +115,7 @@ etcdctl \
   --ca-file=/opt/k8s/work/ca.pem \
   --cert-file=/opt/k8s/work/flanneld.pem \
   --key-file=/opt/k8s/work/flanneld-key.pem \
-  mk ${FLANNEL_ETCD_PREFIX}/config '{"Network":"'${CLUSTER_CIDR}'", "SubnetLen": 21, "Backend": {"Type": "vxlan"}}'
+  mk ${FLANNEL_ETCD_PREFIX}/config '{"Network":"'${CLUSTER_CIDR}'", "SubnetLen": 24, "Backend": {"Type": "vxlan"}}'
 ```
 + flanneld **当前版本 (v0.11.0) 不支持 etcd v3**，故使用 etcd v2 API 写入配置 key 和网段数据；
 + 写入的 Pod 网段 `${CLUSTER_CIDR}` 地址段（如 /16）必须小于 `SubnetLen`，必须与 `kube-controller-manager` 的 `--cluster-cidr` 参数值一致；