use docker's overlay2 storage driver on Ubuntu 16.0+

Docker defaults to using its aufs storage driver on Ubuntu 16.0.4, but the system has a 4.4 kernel, so it's probably a good idea to switch over to the overlay2 driver.

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 16.04.2 LTS
Release: 16.04
Codename: xenial

$ uname -r
4.4.0-83-generic

$ sudo cat /etc/docker/daemon.json
{
  "storage-driver":"overlay2",
  "log-driver": "journald"
}