Normally, machine ID is generated randomly when dnf installs @Core
group. Unfortunately this isn't helping us with reproducibility
of images.
This commit introduces the concept of fake machine ID. Before dnf
command is run in dnf stage, we set the machine ID to a fake one.
This ensures all the scriptlets requiring machine ID have predictable
outputs.
For example GRUB uses machine-id in file names inside
/boot/loader/entries. With fixed machine ID the file names are always
the same and totally predictable.
cc73fa5d10