Support for sections in .conf [JIRA: RIAK-3054]

It might be useful to expand the config file parser to support unix-conf-style sections, e.g.:

```
[foo.bar]
a = 3
b = test
```
instead of
```
foo.bar.a = 3
foo.bar.b = test
```

The semantics could be really simple: Every key `$key` found in section `[$section]` is prefixed with `$section`, resulting in `$section.$key`. 

This would help to avoid redundancy in the `.conf` files.

