Ansible: Use the overlay2 Docker storage driver.

The btrfs driver is buggy (see
https://github.com/moby/moby/issues/27653). I've had to deal with this
several times now and mostly on ns4 (where most of the CI jobs run).
Keep the btrfs partitions (snapshots make backups easier) but use the
`overlay2` storage driver to avoid to dangling subvolumes.