Remove SSH dependency by automatically clustering nodes

Docker was designed to run a single process when it is launched. Before this pull request, Riak and an SSH daemon were launched so that Riak nodes could be clustered from the container host via SSH.

This pull request removes SSH as a dependency and joins nodes into a cluster as they come up. After the fifth node joins the cluster, it issues a `riak-admin cluster plan` and `riak-admin cluster commit` to finalize the clustering process.

### Trade-offs

- Spinning up a Riak cluster is easier
- SSH and `sshpass` are no longer dependencies
- Riak configuration changes must be done via a `Dockerfile`
- Access to Riak via `riak-admin` goes away
- Cluster size is currently fixed (`5`)