cinder db purge util

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

This is a useful feature as Openstack matures and operators/admins are upgrading over time.