Add environment variables to aggregate service containers

[bfirsh]

E.g. an environment variable for all redis hosts, comma separated.

[dnephin]

Not sure what this is about, @bfirsh got more context, or should we close this?

[bfirsh]

The idea was that if you scaled up a service, you'd get an environment variable with something like REDIS_HOSTS=10.0.0.1,10.0.0.2,10.0.0.3. I think this is probably a bit ancient though, and we can come up with better ways of doing this in context of new networking etc.