This fixes containers#1457

Co-authored-by: Valentin Rothberg <vrothberg@redhat.com>
Signed-off-by: Dan Čermák <dcermak@suse.com>

Currently we would display an error when the user does not specify a `driver` in
their config file. This has been present for historical reasons mostly to
prevent users from accidentally getting the vfs
driver (containers#1460 (comment)). Now
that most systems support the overlay driver natively, we can reduce this to a
warning and only warn about it if the driver_priority list is unset. If it is
provided, then clearly the user or the distribution wanted for c/storage to pick
a driver itself and the warning would be only confusing to users.

Signed-off-by: Dan Čermák <dcermak@suse.com>