fix installation with JBOD: set some grub-installer param

Set grub-installer/only_debian and grub-installer/with_other_os
to false since the conflict with the settings for grub-installer/bootdev
done in partman/early_command.

Fixes: SMBCAP-3648

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>