On destroy, retain the DynamoDB table by default (if it has anything in it).

[zanerock]

Overview

We don't want to accidentally delete important data.

Deliverables

• figure out how to retain the DB and make sure it doesn't cause a false error when it fails to delete
• implement --destroy-data option that will delete the DB
• check if DB is empty, in which case we always delete it
• add a retained section to cloudsite-db.json which is like cleanup described in Implement delayed cleanup support #93, except it lists retained items.
• implement cloudsite retained list, cloudsite retained list --arn which can reuse the logic from cleanup
• implement cloudsite retained delete XXX [YYY...] which deletes the specified resources reusing the logic from cleanup