Add support for HAProxy

This commit adds support for [HAProxy](http://www.haproxy.org/) via the `DOCKER_RIAK_CS_HAPROXY` environmental variable. When set, an additional container will be started with HAProxy. All Riak CS containers in the cluster will be linked to the HAProxy container, providing one endpoint that load balances requests evenly against all Riak CS nodes.

Aims to resolve https://github.com/hectcastro/docker-riak-cs/issues/6.