[atomic] How to migrate a Fedora Atomic Host from devicemapper to overlay2

[Colin Walters <walters verbum org>]

There's some work on `atomic storage migrate`:
https://trello.com/c/vAunYr5K/310-docker-storage-migrate-images-containers-when-switching-drivers

However, if you just have a fresh system and are OK blowing away
all of your containers/volumes:

```
$ systemctl stop docker
$ rm /var/lib/docker/* -rf
$ lvm lvremove atomicos/docker-pool
  # Reallocate space to the root - tweak how much to your liking
$ lvm lvextend -r -L +50%FREE atomicos/root
$ echo STORAGE_DRIVER=overlay2 >> /etc/sysconfig/docker-storage-setup
$ rm /etc/sysconfig/docker-storage
$ systemctl start docker
```