Ensure riak_cs_kv_multi_backend is installed before starting Riak

At one point Riak init scripts would fail silently on start if there was a configuration error. Since that issue was resolved in our packages, a [hack](https://github.com/basho/vagrant-riak-cs-cluster/commit/9debb64c92b6ba2e21f287987bd1320482bb4e9e) was introduced so that this project could work around it.

This PR resolves the issue a little better by leveraging a [modularized](https://github.com/basho/riak-cs-chef-cookbook/pull/21) Riak CS cookbook.

**Note**: Please do not merge until https://github.com/basho/riak-chef-cookbook/pull/84 and https://github.com/basho/riak-cs-chef-cookbook/pull/21 are merged and tagged appropriately.