Turn off delete_automated_backups on RDS instances

The delete_automated_backups option in terraform's RDS resource defaults to true, so when the DB is deleted any point-in-time auto backups are deleted as well. For Dedicated, we would very much like that to be able to be 'false', so that if terraform is ever involved in deleting an RDS instance inadvertently we can restore from retained backups.

Given GET has no other opinions about backups and in particular doesn't set up any automated backups, there's a case to be made each of:

1. Setting to false all the time (force the safer option for anyone who has gone to the trouble of setting up automated backups, without affecting anyone who doesn't)
2. Configurable, defaulting to false (as for (1), but can be turned back to true if desired)
3. Configurable, defaulting to true (current behavior by default, so not as safe by default but the least change (costs) for existing deployments).

For Dedicated needs any of those 3 is acceptable, but I'd tend to prefer 1 or 2 rather than 3, on the grounds that not retaining the backups was a surprising default behavior to me and is likely to affect users of GET in the worst way at the worst time when they thought they had backups but find they don't. @grantyoung before I go as far as rustling up an MR, could you let me know which of those options you'd prefer me to implement?