Data bag, multiple buckets and recipe modernization

This PR has 3 distinct improvements to this recipe:

1. Move credentials to a data bag from a node attribute. It is a Chef best practice to place service credentials such as S3 API keys in a data bag. (I did not also adopt encrypted data bags at this time, however.)
2. Support mounting of multiple buckets. This is done via a bucket list in the credential data bag, but could as easily live as a node attribute if preferred.
3. Modernize the recipe (metadata.rb not .json) and resolve [foodcritic] complaints for style.

There's a lot more that can be done with this recipe, namely picking up some of your forks' providers/LWRPs for mounting buckets and separating bucket mounting from s3fs installation, however I wanted to get this work upstream first to gauge your opinion on the matter.

[foodcritic]: http://acrmp.github.io/foodcritic/