Expose S3 configurations in the config object

I'm adding support to configure the S3 bucket and hosts when installing Ruby versions.

After [these changes](https://github.com/boxen/puppet-ruby/compare/6.5.0...c545a00) `rbenv install` is downloding from the configure bucket, but this is not the same when we install Ruby versions using `ruby::version` class since it execute the `rbenv install` command in a separated shell.

To make it work I need to expose these environment variables in puppet Facts to se the environment variables correctly on the `ruby::version` class.

cc @wfarr @boxen/owners