deployment: delete old database periodically

[derhuerst]

Currently, the build script tries to delete the old/previous database right away. This fails in production, because the service is running, so PostgreSQL won't delete it.

Instead, there should be a timed task in the deployment setup repo that periodically prunes all databases but the n most recent.

• write cleanup script
• remove deletion/cleanup part from build.sh
• integrate cleanup script into digitransit-ansible