Suppress dumping of whole configuratinos by default

Fluentd dumps whole configuration content into log by default (`info` log level). It can be dangerous when a plugin includes secret data.

There's actually such a plugin, for example, fluent-plugin-secure-forward. With the plugin, fluentd dumps configuration like below:

```
...

<match secure_forward.**>
  type secure_forward
  shared_key xxxxxxxxxxxxxxxx
  self_hostname app.example.com
  <server>
    host log.example.com
    username foo
    password xxxxxxxxxxxxxxxx
  </server>
</match>

...
```

We had better not to expose secret data in such a place where people don't think their secret data can be.