edxapp-migrate-lms figures out at system build time what databases
it needs to migrate (just default, or default + student_module_history).
This avoids paver needing to be clever about figuring out if a user has
enabled the split CSMHE databases flag and permits this to merge early
and safely.

We can turn the CSMHE flag on from configuration at the same time we
enable all of the database config and ensure that the new database is
created.  Until a devstack is provisioned using that version of
configuration, it'll continue to have 1 DB and migrate 1 DB.  This
allows us to better control deployment.