Revert "image: Switch to ostree-format: oci"

[cgwalters]

This reverts commit 7a4d1bc.

Need to fix the rhcos upgrade test in coreos-assembler.

[miabbott]

For context, the failure looks like:

    --- FAIL: rhcos.upgrade.basic (51.10s)
    --- FAIL: rhcos.upgrade.basic/setup (6.96s)
            cluster.go:143: error: opening repo: opendir(objects): No such file or directory
            cluster.go:191: "sudo ostree --repo=/sysroot/ostree/repo pull-local /var/srv 53d66cde042c44ec5c33262a1c4be46fbd423289ab2d52e87af2ced5f19ff399 && sudo rm -rf /var/srv/* && sudo sync" failed: Process exited with status 1; logs from journalctl -t kola:
    + sudo tar -xf rhcos-49.84.202109011424-0-ostree.x86_64.ociarchive -C /var/srv && sudo rm rhcos-49.84.202109011424-0-ostree.x86_64.ociarchive
    + sudo ostree --repo=/sysroot/ostree/repo pull-local /var/srv 53d66cde042c44ec5c33262a1c4be46fbd423289ab2d52e87af2ced5f19ff399 && sudo rm -rf /var/srv/* && sudo sync
    --- SKIP: rhcos.upgrade.basic/upgrade-from-previous (0.00s)
            cluster.go:53: A previous test has already failed
    --- SKIP: rhcos.upgrade.basic/verify (0.00s)
            cluster.go:53: A previous test has already failed
    --- PASS: rhcos.upgrade.basic/rootfs-migration (18.55s)
    --- PASS: rhcos.upgrade.basic/verify-rootfs-migration (0.20s)
=== RUN   rhcos.upgrade.luks/setup
=== RUN   rhcos.upgrade.luks/upgrade-from-previous
=== RUN   rhcos.upgrade.luks/verify
--- FAIL: rhcos.upgrade.luks (74.46s)
    --- FAIL: rhcos.upgrade.luks/setup (8.78s)
            cluster.go:143: error: opening repo: opendir(objects): No such file or directory
            cluster.go:191: "sudo ostree --repo=/sysroot/ostree/repo pull-local /var/srv 53d66cde042c44ec5c33262a1c4be46fbd423289ab2d52e87af2ced5f19ff399 && sudo rm -rf /var/srv/* && sudo sync" failed: Process exited with status 1; logs from journalctl -t kola:
    + sudo tar -xf rhcos-49.84.202109011424-0-ostree.x86_64.ociarchive -C /var/srv && sudo rm rhcos-49.84.202109011424-0-ostree.x86_64.ociarchive
    + sudo ostree --repo=/sysroot/ostree/repo pull-local /var/srv 53d66cde042c44ec5c33262a1c4be46fbd423289ab2d52e87af2ced5f19ff399 && sudo rm -rf /var/srv/* && sudo sync
    --- SKIP: rhcos.upgrade.luks/upgrade-from-previous (0.00s)
            cluster.go:53: A previous test has already failed
    --- SKIP: rhcos.upgrade.luks/verify (0.00s)
            cluster.go:53: A previous test has already failed
FAIL, output in tmp/kola-upgrade
Error: harness: test suite failed
2021-09-01T14:58:02Z cli: harness: test suite failed

[miabbott]

/lgtm

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[cgwalters]

Ugh, and trying to hack on the coreos-assembler upgrade test for this, until rhcos gets a new enough rpm-ostree that speaks this directly, we'll need to do the conversion outside the os.