Use `dotted` notation for configuration attributes as well

This commit makes it possible to use either underscored symbol keys for
configuration attributes (i.e. attributes defined under `broker`), or to
use the `dotted` notation, i.e.

```
node.set.broker.log_dirs = %w[/some/dir-1 /some/dir-2]
node.set.broker.log.dirs = %w[/some/dir-1 /some/dir-2]
node.set.broker = {‘log.dirs’ => %w[/some/dir-1 /some/dir-2]}
```

Are all valid ways to set attributes that will be used when rendering
the broker configuration file. Got rid of quite a lot of unit tests that
simply aren’t needed anymore, yay.