watcher provides the CLI tool to purge delete or orphaned objects from
database[1]. This allows users to set up cron job to periodically
execute the CLI to reduce database size.
[1] https://docs.openstack.org/watcher/latest/man/watcher-db-manage.html#purge
Change-Id: I70fe945818fdd4a31ddd0d0d502df53f6eeb5b3f