zhuyifei1999@toolsbeta-worker-1001:~$ curl https://toolsbeta-flannel-etcd-01.toolsbeta.eqiad.wmflabs:2379/v2/keys/?recursive=true; echo
{"action":"get","node":{"dir":true}}

[A long time of digging around...]
zhuyifei1999@toolsbeta-worker-1001:~$ curl https://toolsbeta-flannel-etcd-01.toolsbeta.eqiad.wmflabs:2379/v2/keys/coreos.com/network/config -XPUT -d value='{ "Network": "192.168.128.0/17",  "Backend": { "Type": "vxlan" } }'
{"action":"set","node":{"key":"/coreos.com/network/config","value":"{ \"Network\": \"192.168.128.0/17\",  \"Backend\": { \"Type\": \"vxlan\" } }","modifiedIndex":5,"createdIndex":5}}
zhuyifei1999@toolsbeta-worker-1001:~$ curl https://toolsbeta-flannel-etcd-01.toolsbeta.eqiad.wmflabs:2379/v2/keys/?recursive=true | jq .
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  1267  100  1267    0     0  22778      0 --:--:-- --:--:-- --:--:-- 23036
{
  "action": "get",
  "node": {
    "dir": true,
    "nodes": [
      {
        "key": "/coreos.com",
        "dir": true,
        "nodes": [
          {
            "key": "/coreos.com/network",
            "dir": true,
            "nodes": [
              {
                "key": "/coreos.com/network/config",
                "value": "{ \"Network\": \"192.168.128.0/17\",  \"Backend\": { \"Type\": \"vxlan\" } }",
                "modifiedIndex": 5,
                "createdIndex": 5
              },
              {
                "key": "/coreos.com/network/subnets",
                "dir": true,
                "nodes": [
                  {
                    "key": "/coreos.com/network/subnets/192.168.215.0-24",
                    "value": "{\"PublicIP\":\"10.68.18.110\",\"BackendType\":\"vxlan\",\"BackendData\":{\"VtepMAC\":\"ce:b0:30:4d:62:48\"}}",
                    "expiration": "2018-05-09T02:17:23.548978467Z",
                    "ttl": 86392,
                    "modifiedIndex": 6,
                    "createdIndex": 6
                  },
                  {
                    "key": "/coreos.com/network/subnets/192.168.130.0-24",
                    "value": "{\"PublicIP\":\"10.68.20.72\",\"BackendType\":\"vxlan\",\"BackendData\":{\"VtepMAC\":\"ba:57:9d:85:c2:e4\"}}",
                    "expiration": "2018-05-09T02:17:23.982088555Z",
                    "ttl": 86392,
                    "modifiedIndex": 7,
                    "createdIndex": 7
                  },
                  {
                    "key": "/coreos.com/network/subnets/192.168.131.0-24",
                    "value": "{\"PublicIP\":\"10.68.22.202\",\"BackendType\":\"vxlan\",\"BackendData\":{\"VtepMAC\":\"fa:3b:36:2a:31:49\"}}",
                    "expiration": "2018-05-09T02:17:24.453518564Z",
                    "ttl": 86393,
                    "modifiedIndex": 8,
                    "createdIndex": 8
                  }
                ],
                "modifiedIndex": 6,
                "createdIndex": 6
              }
            ],
            "modifiedIndex": 5,
            "createdIndex": 5
          }
        ],
        "modifiedIndex": 5,
        "createdIndex": 5
      }
    ]
  }
}