goreman: Support initialization stages

Created by: efritz

Update the internal/goreman package to support calling Start multiple times in order to add processes to an already running system on subsequent invocations.

This will be necessary to run migrations in the all-in-one Docker image, which will require postgres to be up (possibly running in goreman), then the migrator to run, then all other services to run.

We then can separate two clusters of services: Postgres (and related) lines, then the remaining services.