Move from using 'zram' to 'zram-generator-defaults' in the ostree bootiso
testing manifest. More information is available in Fedora 33 Change
document [1].
Add org.osbuild.kernel-cmdline stage to fedora-boot.json manifest
because of change in how grub handles the kernel command line arguments
[2].
GRUB2 Stage 2 checksums in assemblers test are updated. The change have
been verified by building the fedora-boot.json manifest with each checked
filesystem and booting the image in QEMU with legacy mode.
[1] https://fedoraproject.org/wiki/Changes/SwapOnZRAM
[2] https://github.com/osbuild/osbuild-composer/pull/982#issuecomment-697356929
Signed-off-by: Tomas Hozza <thozza@redhat.com>