Add a riak_kv_multi_backend case for attribute configuration

As referenced in https://github.com/basho/riak-chef-cookbook/pull/72, it would be useful to have a case for when users select the `riak_kv_multi_backend` backend. If the `riak_kv_multi_backend` backend is selected, Bitcask and eLevelDB backends are created, with Bitcask as the default.

See: http://docs.basho.com/riak/latest/tutorials/choosing-a-backend/Multi/#Configuring-Multiple-Backends

/cc @jeremyong