Take advantage of the kitchen-vagrant Vagrantfile template hook

[kitchen-vagrant](https://github.com/test-kitchen/kitchen-vagrant) `v0.12.0` added support for providing a custom `Vagrantfile` [ERB](http://ruby-doc.org/stdlib-1.9.3/libdoc/erb/rdoc/ERB.html) template. This allows us to enable a variety of Vagrant plugins during the test-kitchen runs.

In addition, this pull request adds test-kitchen support for CentOS 5.9

- [x] Test Riak CS on CentOS 5.9
- [x] Test Riak CS Enterprise on CentOS 5.9