Convert _ALL_ configuration attributes to a flat namespace

Oh this is just the beginning. FULL SPEED AHEAD!
Put all configuration attributes under a `broker` namespace

Next stop, dynamically generate configuration properties.
Generate property keys from `broker` attributes

Instead of defining _ALL_ possible configuration attributes in
`attributes/default.rb` we’ll generate key-value pair from whatever
attributes that are found under the `broker` namespace.

The diffs in this commit are so _HORRIBLE_, but I couldn’t think of any
other way to get this done. There are still some tests that needs to be
rewritten / added, and I do believe that the configuration generation
creates some unnecessary whitespace / newlines that needs to be dealt
with, but everything has it’s time, mkay?

This is going to be so much easier to maintain.
Allow `dotted` notation for broker configuration

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

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

Decided to get rid of support for using nested Hashes as it’ll become
difficult to render Hash-based options (CSV strings) correctly, and due
to the naming of some of the configuration parameters it’s not possible
to use `node.default.this.attribute = ‘something’` as the parameters
include `default`, `fetch` and whatever else.

Removed a lot of unit tests that are unnecessary now that the broker
configuration file is dynamically rendered.
Leave “mixed” configuration names as-is

If there’s a configuration parameter that contains both underscore and
dots we don’t want to convert underscores to dots (for obvious reasons).
Allow nested Hash notation for broker configuration

So, decided to re-introduce the nested Hash notation for accessing
configuration attributes, as such:

	`node.default.broker[:hash][:option] = ‘value’`

It does however mean that the “per topic” attributes cannot use Hashes
anymore, but either Arrays or just plain Strings.
Update headers in template files

Just a heads up.
Remove unused config. related partials

Not used since the configuration rendering is done using helper methods
rather than partials.
Merge branch 'dynamic-configuration-attributes'