Add accumulator and resource for writing storage-aggregation.conf

First of all, thank you for the awesome rewrite of the cookbook! The accumulator pattern to generate the INI files is fantastic.

This essentially copies the functionality of the resources/providers that write out `storage.conf`, in order to provide the same pattern to write out `storage-aggregation.conf`. I found myself in need of this tonight so that I could configure `storage-aggregation.conf` [for Statsd](https://github.com/etsy/statsd/blob/master/docs/graphite.md).

Let me know if you'd like any additional testing for this, or if I'm completely off! I would also be happy to write accumulator resources for the other configuration files that aren't covered yet (in which case some additional DRYing of the accumulator code might be in order as well).