tests/fetch_image: add SKIP_FETCH_IMAGE env var
Allow skipping the image fetch logic to let the gerrit job copy the
image from another job instead.
Related: osmo-ci I736b2b194813e6edff0197145fe08168c3e66903
Change-Id: I9445d6d5e4723bf6a87d4bffcdc638560471328b
