Set the default to cos-stable-65 (which is what we are using on GKE for
latest 1.9 and 1.8) and set config-test to use cos-beta-66, so that we
can get more exposure to it.
The testgrid seems to be fairly happy with these images. (both
e2e-gce-cosdev-k8sdev-default and e2e-gce-cosbeta-k8sdev-default are
generally green.)