Add support for HAProxy

This commit adds support for HAProxy 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.
Add error message if HAProxy is enabled without automatic clustering.
Merge branch 'feature/haproxy' into develop
0.5.0
Merge branch 'release/0.5.0' into develop