Switch to overlay2 driver for storage

Switch to overlay2 driver for storage

This patch switches the default to the overlay2 storage driver and see
if it helps performance.

Background:

The loopback driver is not recommended for production. Most
other docker storage backends require extra disks (or partitions)
which we don't have on the root disk. Overlay seems to make the
most since for TripleO upgrades where we intend to update
in-place installations to use docker.

Co-Authored-By: Martin André <m.andre@redhat.com>
Change-Id: I6896a9b3e9dc3e269bf5b0dc753bf8c985482daf