Allow changing Riak backend

This PR allows changing the Riak backend.  From the looks of it though, you'll only be able to change it once because the sed expression in `riak.sh` won't find bitcask after the first time a cluster is stood up.  That may be ok.  Fixes #21

Allow always setting the storage backend

Fix setting default backend

Allow always setting the storage backend

Fix setting default backend
Merge branch 'jp-riak-backend' of github.com:jplock/docker-riak-1 into jp-riak-backend