Better db-migrate

Maybe it's better?

This lets you define your database parameters just like any other app parameter, but it removes the notion of different database environments (at least for now).

It also puts in place `test/migrations.test.js`, but with no actual test yet. The actual tests would look a lot like [this](https://github.com/mozilla/openbadges/blob/development/test/migration.test.js#L91), but adapted to mocha/should instead of node-tap.