Support for Namenode auto failover

Hi, would you mind if add support for standby namenode ?

I'm using this out_webhdfs plugin to transfer data into a CDH4-based cluster where the namenode auto-failover feature has been enabled. I want to handle the case that when the  auto-failover occurs, but as you may know, [WebHDFS currently dont't support HA configuretion](https://issues.cloudera.org/browse/DISTRO-403). So I think maybe we can solve this by adding a standby_namenode support in this plugin.

If the 'standby_namenode' directive has been set,
the plugin will automatically switch to use standby namenode under any
of the following scenes:
- Try to write the data to HDFS but got an 'org.apache.hadoop.ipc.StandbyException' exception
- HDFS-write failed >= 11 times ( retry 11 times costs fluentd about 1023s,  I guess it's long enough to exclude other failure reasons like temporary network problem or single datanode down )

Of course, use case without standby_namenode won't be infected.