Manifest: always set kernel options in grub2 stage

It turned out that when we stopped setting the kernel options in grubenv
file, we also stopped setting them in /etc/default/grub under
`GRUB_CMDLINE_LINUX`. This file is used by grub2-mkconfig when
generating grub configuration.

10_linux script executed by grub2-mkconfig recently started to overwrite
the /etc/kernel/cmdline, if its timestamp is older than the timestamp of
/etc/default/grub [1]. As a result, all kernel options were wiped out from
/etc/kernel/cmdline.

Make sure that we always set the `KernelOptions` in the grub2 stage
options, even if the `WriteCmdLine` is set to `false`.

In addition, unify the way we concatenate kernel options set in the
grub2 stage options. Some pipeline implementations were previously using
space, other were using comma. Space is now used everywhere.

Regenerate all affected image manifests.

[1] https://src.fedoraproject.org/rpms/grub2/c/fc76aed5333f56dd05400521a35b944a5df52ebc

Signed-off-by: Tomáš Hozza <thozza@redhat.com>

test/data/manifests/fedora_39-x86_64-qcow2_customize-boot.json

@@ -5590,6 +5590,7 @@
             "options": {
               "root_fs_uuid": "6e4ff95f-f662-45ee-a82a-bdf44a2d0b75",
               "boot_fs_uuid": "0194fdc2-fa2f-4cc0-81d3-ff12045b73c8",
+              "kernel_opts": "ro no_timer_check console=ttyS0,115200n8 biosdevname=0 net.ifnames=0 debug",
               "legacy": "i386-pc",
               "uefi": {
                 "vendor": "fedora",