Allow Riak package installations from custom repositories

Some users prefer to install Riak from their own repositories. This pull request adds a recipe so that users can leverage existing internal repositories without breaking backward compatibility.

See https://github.com/basho/riak-chef-cookbook/pull/67 for details.

- [x] Test open source Riak package installations
- [x] Test Riak Enterprise package installations

/cc @tjarvstrand