[8.0] [ADD] module database_cleanup

This pull request adds a module to clean up after a migration.

Original proposal on Launchpad:
https://code.launchpad.net/~therp-nl/openupgrade-tools/7.0-add-database_cleanup/+merge/203633

The original authors are @StefanRijnhart and @guewen. The migration to 8.0 has been made by @sbidoul.


Put description in README.rst file