Backups are given snapshot IDs but do not delete

This is essentially connected to [#495](https://github.com/digitalocean/doctl/issues/495), after using the newest version (1.22.0), I was still noticing the snapshots were not being deleted. After much work to find the RCA, I found the backups are part of the snapshot list in the API, however although there is no error when attempting to delete them, they are not deleting. That seems to be their nature from the web console, as no delete option is mentioned. This is causing confusion in scripting the deletions, in my situation I need to save the most recent 5 snapshots and delete the rest. Since they are filtered as one list with the snapshots, the wrong snapshots were being deleted. The backups should have a separate ID system, essentially I need a monthly backup and daily snapshots, with the current system this is tedious. My script is attached.

[do snapshots.zip](https://github.com/digitalocean/doctl/files/3414684/do.snapshots.zip)