CoreOS: tweak rpmostree_rebase test to work from OCI remote
    
    The rpmostree_rebase test has been failing on CoreOS 40+ for a
    while. Per
    https://github.com/coreos/fedora-coreos-tracker/issues/1672
    it turns out this is because FCOS actually deploys OCI remotes
    by default now. Rebasing from an OCI remote to an ostree
    remote (as we are trying to do here) requires specifying the
    registry, so let's do that.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>