Since https://review.opendev.org/c/openstack/tempest/+/923357
there is a new config image parameter which requires a qcow2 image.
Populate it (even in case it's not going to be used) using
the same value of DEFAULT_IMAGE, which happens to be qcow2,
trying to balance reducing duplication of the definition
of external resources (cirros images) and the need to have
a separate variable to track this default.
Change-Id: I01187f061b809b4beb29ae5d96e16325052619ed