Automatic merge from submit-queue

GCI: Fix the condition for using the default image

This PR revises the condition for using the default GCI image. The old logic is not convenient for manually run e2e tests in some cases (mainly for GCI team to test custom images). The new logic by this PR is very similar to the logic in using ContainerVM. When setting distro to "gci", if master or node image is unset, we use gci-dev for it. If either is set, we respect it.

@roberthbailey @zmerlynn @dchen1107 please review it, and we should cherry pick it in release-1.2 branch. Thanks!

cc/ @kubernetes/goog-image @adityakali FYI