Service-to-service network discovery

[lukehoban]

For Services defined outside of Pulumi, we may want a mechanism for simple in-cluster service dsicovery along the lines of Docker links or Kubernetes proxies.

There are several reasons this might not actually be appealing:

1. We already have direct service discovery via closure capture within our Pulumi model, and it's a shame to have to invent another mechanism for purely-outside-of-Pulumi cases
2. It's still possible to pass in references via env vars or similar, which would help make dependencies more explicit anyway.
3. We don't (in our current model) have a private network or in-cluster DNS like Kubernetes, so our options here aren't as clean (see Networking #67).

But all that said - we may decide we still want to do something. We'll need to flesh out use cases more to understand if and where there is real friction in the current experience.

[lukehoban]

If we do this we'll likely do it via #441, so closing this one out and we'll track there.