The MoveUnshardedPolicy is used only in our testing infrastructure and should perform random moveCollection over all the existing collections.

In order to make the selection of the collection random, we need to:

1. select a random database
2. select a random collection in the database

Currently, we only randomize the selection of the collection, but not the one of the databases.

As part of this work we should also update jstests/sharding/balancer_should_return_random_migrations_failpoint.js test to make sure that in the presence of multiple databases the balancer will eventually move all collections.