design a way of handling inventory in menshen

Until now, we've used the static file passed by float (v3 eip-service.json).

We might need now a more generic way of specifying the inventory, and possibly one that is dynamically managed.

This is tied to orchestration-agnostic deployment debate. Ideally we can keep it simple, but perhaps it's good to design an interface that can be replaced by different inventory provider (as an example, I can conceive that small deployments can just dynamically reload a yaml file coming from float hosts, or update a bucket allocation database, while others might want to use something like consul...)