Cache databases to speed up acceptance tests

Currently, our acceptance tests always run every migration to initialize the test database.  This PR allows the tests to cache the schema and migration history in the git repository.  This has two advantages:

1) The acceptance tests complete about 1 minute faster.
2) We can avoid the "OpenID" workaround when using MySQL for the bok-choy acceptance test suite (see [this step in the edxapp deploy scripts](https://github.com/edx/configuration/blob/d1bc29c72b4a1bae0fb19c369add09f643d0717d/playbooks/roles/edxapp/tasks/deploy.yml#L251-L255) for details).

When developers make changes to the database schema and run the tests locally, the database cache will be updated.  This will show up in the git changes, and developers can commit the updated cache to the repo to speed up future test migrations.

Suggested reviewers: @jzoldak @ormsbee