glance db purge util for Glance

Adding functionality to `glance-manage db` to safely purge "deleted" data. This will have non-optional argument of nDays. For example `glance-manage db purge 60` will go into the glance db, and delete from each table where 'deleted =! 0 and deleted_at > date_sub(NOW(), interval 60 day);' This is a useful feature as...