Changes the `configs_for` method from using traditional arguments to
using kwargs. This is so I can add the `include_replicas` kwarg without
having to always include `env_name` and `spec_name` in the method call.

`include_replicas` defaults to false because everywhere internally in
Rails we don't want replicas. `configs_for` is for iterating over
configurations to create / run rake tasks, so we really don't ever need
replicas in that case.