Add support for operating on column **and** database level separated tenants.

[alrik]

Story

As a service operator I would like to run all my freemium tenants on a shared database with tenant separation on column level. All my premium customers data should be stored in a dedicated database, maybe even on another mongodb cluster. I want to have only one app deployment that can manage all my tenants.

Goal

The goal of this issue is to provide some kind of a per-tenant config or config resolver.