Update storage-driver for promotion of overlay2

[thaJeztah]

Problem description

moby/moby#34430 updated the list of storage-drivers and promoted overlay2 over aufs.

As a result, aufs will no longer be selected automatically if the required kernel modules are installed to support it. For comparison;

Before this PR (taking Ubuntu 16.04 as example, because it always has kernel 4+)

• When installing docker on a Ubuntu 16.04 machine with the linux-image-extra package installed (which provides aufs), docker would select aufs as storage driver.
• When installing docker on a Ubuntu 16.05 machine without the linux-image-extra package installed, docker would select overlay2

After this PR, overlay2 will be selected as the default, unless an existing aufs directory was found (so users that are currently running aufs will stay on aufs unless they do a "factory reset" and wipe out their /var/lib/docker directory)

Problem location

Most likely this section needs an update; https://docs.docker.com/engine/userguide/storagedriver/selectadriver/

Project version(s) affected

This change will be included in the next Docker release that is using the current master of the moby/moby repository (17.08)

[AkihiroSuda]

@mstanleyjones @thaJeztah What's current status?