Ecto: Ecto.rollback migrates down more steps than it should inside an umbrella app

[ben-ic]

When running ecto.rollback in an umbrella application in which multiple apps are using the same repo it will rollback more than 1 step (or more than "n" steps if you used -n).

[ericmj]

I think we should make the migration tasks non-recursive and instead discover all unique repos in the umbrella apps and then run migrations on them.