Debian 12 packer template and some minor fixes

[mirwan]

What this PR is about:

• Packer template for Debian 12: Introduced user variables in order to, first, generate httpdir/user-data, then, use the vars in template, and at shutdown, remove the packer user
• Adjust example_build/scripts/pre-install-baremetal.sh
  • For Debian 12 (non-free-firmware)
  • Change the kernel to the distrib default one (to mimic OVH standards apparently)
  • Remove apt-get clean which removes downloaded packages

Note to reviewers:

• I used separate commits so that they can be dropped if judged unworthy (default kernel for example)
• Despite the removal of apt-get clean I couldn't make the image work as is on a kimsufi without efi. I didn't have KVM to debug and ended up with installing grub-pc (without download-only) in example_build/scripts/pre-install-baremetal.sh and using dpkg-reconfigure grub-pc in example_build/files/make_image_bootable.sh

[sbraz]

Hello @mirwan,
Sorry for the delay, we completely missed your PR. I think we addressed some of the issues in #4, could you please try again and update the code if it's still required?