Use <server> option instead of obsoleted host option

I often use `--setup` option to create initial configuration file. The file created by the setup option uses obsoleted `host` option in out_forward, so the following warnins occur when Fluentd starts with the configuration. If there is no reason to keep it, please fix it.
```
2014-03-22 16:38:27 +0900 [warn]: 'host' option in forward output is obsoleted. Use '<server> host xxx </server>' instead.
```