test:
[sig-imageregistry][Feature:ImageAppend] Image append should create images by appending them failing due to inconsistent images between CI and OCP

is failing frequently in CI, see search results:
https://search.ci.openshift.org/?maxAge=168h&context=1&type=bug%2Bjunit&name=4.7&maxMatches=5&maxBytes=20971520&groupBy=job&search=%5C%5Bsig-imageregistry%5C%5D%5C%5BFeature%3AImageAppend%5C%5D+Image+append+should+create+images+by+appending+them


This is a result of OCP images having an extra layer for injecting repository information. This test needs to be updated to handle that the shell image being used can have either 6 or 7 layers depending on whether it's an OCP or origin image.

See: https://prow.ci.openshift.org/view/gs/origin-ci-test/logs/release-openshift-ocp-installer-e2e-aws-4.7/1335987821197922304

fail [github.com/openshift/origin/test/extended/images/append.go:137]: Expected
    <[]v1.ImageLayer | len:6, cap:8>: [
        {
            Name: "sha256:1b8dabac56ed728c17a670d327474ab87dc392dc17721854ea599a7753326579",
            LayerSize: 74453962,
            MediaType: "application/vnd.docker.image.rootfs.diff.tar.gzip",
        },
        {
            Name: "sha256:6500ac87b29ffd00c8655be65a6824dfcf9fc0accc625158ef1060bcedc84ca8",
            LayerSize: 1788,
            MediaType: "application/vnd.docker.image.rootfs.diff.tar.gzip",
        },
        {
            Name: "sha256:d7d8dd57ffd8472e232127c8f3db02403d549d4801797ac7bb195892b4c104f2",
            LayerSize: 679812,
            MediaType: "application/vnd.docker.image.rootfs.diff.tar.gzip",
        },
        {
            Name: "sha256:a1aeb53e1245cf67fff17ffc4ee2407c339a64befe5b15837e3d3719ea17018b",
            LayerSize: 10925616,
            MediaType: "application/vnd.docker.image.rootfs.diff.tar.gzip",
        },
        {
            Name: "sha256:af8f9688a121396d59477c82a72dc360adfb4905a7df63f79a35c1c601adda0a",
            LayerSize: 22277767,
            MediaType: "application/vnd.docker.image.rootfs.diff.tar.gzip",
        },
        {
            Name: "sha256:3f9ba227910551286b8ee1320453ba39ead301de5b5032eea729447771b2b0d4",
            LayerSize: 83393294,
            MediaType: "application/vnd.docker.image.rootfs.diff.tar.gzip",
        },
    ]
to have length 7