Bugfix: Switch to `default` repo layout for the s3 backend

The default layout for the s3 backend is now `default` (instead of `s3legacy`).
Also, there's a new `migrate` command to convert an existing repo, it can be
run like this: `restic migrate s3_layout`

https://github.com/restic/restic/issues/965
https://github.com/restic/restic/pull/1004
