topgun: use `overlay` by default
topgun: use `overlay` by default

it seems like warden does not play well with btrfs - forcing `overlay`
by default makes it possible to run BOSH topgun tests locally.

Signed-off-by: Ciro S. Costa <4f5054ab2db051d03adf9068233a850c82b24437@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse