Fix Riak startup error due to missing riak_cs_kv_multi_backend 

Riak was failing to start due to the missing `riak_cs_kv_multi_backend`. The Riak CS package needs to be installed before Riak is started with that backend configured.

The `erlang_template_helper` was also removed. It was only used once in the project, and its use was replaced with a string literal that converts the Ruby array to an Erlang tuple (https://github.com/basho/vagrant-riak-cs-cluster/commit/643a85514a0fc4f028abf964d78643b6258e83bb#L3L21).