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
I tested this `sed` expression locally and it appears to work regardless of the configured backend:

```bash
sed -i.bak "s/storage_backend, \(.*\)}/storage_backend, ${DOCKER_RIAK_BACKEND}}/" /etc/riak/app.config
```

Example output:

```bash
vagrant@riak-01:~$ export DOCKER_RIAK_BACKEND=riak_kv_eleveldb_backend
vagrant@riak-01:~$ sudo sed -i.bak "s/storage_backend, \(.*\)}/storage_backend, ${DOCKER_RIAK_BACKEND}}/" /etc/riak/app.config
vagrant@riak-01:~$ diff /etc/riak/app.config /etc/riak/app.config.bak
83c83
<             {storage_backend, riak_kv_eleveldb_backend},
---
>             {storage_backend, riak_kv_bitcask_backend},
vagrant@riak-01:~$ export DOCKER_RIAK_BACKEND=riak_kv_bitcask_backend
vagrant@riak-01:~$ sudo sed -i.bak "s/storage_backend, \(.*\)}/storage_backend, ${DOCKER_RIAK_BACKEND}}/" /etc/riak/app.config
vagrant@riak-01:~$ diff /etc/riak/app.config /etc/riak/app.config.bak
83c83
<             {storage_backend, riak_kv_bitcask_backend},
---
>             {storage_backend, riak_kv_eleveldb_backend},
```