HA configuration performs incorrectly

Problem:
I am running HDFS within my Mesos cluster. It is fully HA. I have configured a matcher to point to both NameNodes. However, when the first listed NameNode is in standby mode, the standby_namenode is never used.

Expected behavior:
Connection to the `namenode` NameNode succeeds, finds its in `standby` mode, and attempts to send to `standby_namenode` which is now the active NameNode.

Actual results:
```
2018-06-12 19:28:48 +0000 [warn]: #0 [out_webhdfs] webhdfs check request failed. (namenode: name-0-node.hdfs.mesos:9002, error: {"RemoteException":{"exception":"StandbyException","javaClassName":"org.apache.hadoop.ipc.StandbyException","message":"Operation category READ is not supported in state standby. Visit https://s.apache.org/sbnn-error"}})
```

This is using td-agent 3.1.1 (fluentd 1.0.2) with the shipped fluent-plugin-webhdfs 1.2.2 plugin.

Forcing a NameNode failover caused logs to start flowing, again. However, this required manual intervention and I think the driver should do the correct thing in this state.