Use more env vars for config

It will make it easier to test federation

migrations/2018-05-02-113930_drop_instance_local_domain/down.sql

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+-- This file should undo anything in `up.sql`
+ALTER TABLE instances ADD COLUMN local_domain VARCHAR NOT NULL;

migrations/2018-05-02-113930_drop_instance_local_domain/up.sql

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+-- Your SQL goes here
+ALTER TABLE instances DROP COLUMN local_domain;