Add mongodb adapter stubs.

[rhetzler]

This allows us to synthetically "create" mongodb tenant dbs, unblocking the index migrations.

In practice this is a no-op for create. Drop needs to plug into Mongo.Ecto functionality relying on the more recent database switching functionality (ecto-2.1 branch)

[Dania02525]

I like it! how do you feel about rescuing exceptions in the error tuple format from the mongo adapter for good measure?