Allow multiple instances to use same S3 bucket for backups?

[mhkeller]

@abelsonlive what's the advantage to setting it up as one s3 bucket per instance vs something like setting a string in config.yaml as something like s3_backup_group and constructing the s3cmd put command to something like s3://{{conf.s3_bucket}}/backups/{{conf.s3_backup_group}}/

[mhkeller]

On restore-backup branch the changes are

https://github.com/newslynx/automation/commit/9a540d48cb84e2c1fdf348227d24d7eb28d67ab5#diff-5575782dd113fd040f4b49ec3898f238R9

and here: https://github.com/newslynx/automation/commit/9a540d48cb84e2c1fdf348227d24d7eb28d67ab5#diff-4f0e17ca799f63e1b2d7dbc656c4ee97R30

If there aren't unforeseen problems I can merge to master

[abelsonlive]

Yeah, that's a good idea. Looks fine to me.

[mhkeller]

Merged to master