fix validate_images check in airgap_linter (#2669)
fix validate_images check in airgap_linter (#2669)

Prior to this PR, the check would fail only if the last image happened to be "bad", otherwise bad_image = False would be reset at every loop iteration.
Also modified regex to work with quoted images.
mesosphere/dcos-commons,mesosphere/dcos-commons,mesosphere/dcos-commons,mesosphere/dcos-commons,mesosphere/dcos-commons