Make rpm-ostree tests generic, run on Silverblue and CoreOS
    
    This makes the two rpm-ostree tests written for IoT - overlaying
    and rebasing - work across all rpm-ostree-based flavors we
    currently test (IoT, CoreOS and Silverblue) and runs them on
    all those flavors.
    
    This requires some other changes. For the Workstation ostree
    installer update tests, we have install_default_update_ostree
    upload a disk image and run these tests on that image. That means
    install_default_update_ostree cannot use a scratch disk (as if
    we boot it with two disks but only upload one, the subsequent
    tests fail to boot, looking for the missing second disk), but its
    specified disk size should be large enough for all updates.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>