need to un-fork

As discussed, we'll need to un-fork this repo and instead make it a repo that uses kanod-image-builder.

@pierrecregut shared this example with me:

https://gitlab.com/Orange-OpenSource/kanod/rke-machine

and there is also this example: https://gitlab.com/Orange-OpenSource/kanod/image-builder/-/tree/master/demo?ref_type=heads

What needs to be done, as discussed with Pierre is:

• have the same directory structure in this repo as under the demo directory (or rke-machine, same thing)
  • except that instead of a kanod_demo or rke_machine directory, we have our sylva-specific module -- I'd proposed to name it sylva_imagebuider
• under sylva_imagebuider/elements we'll put what today is under https://gitlab.com/sylva-projects/sylva-elements/diskimage-builder/-/tree/main/kanod_image_builder/elements , but removing everything that isn't specific to Sylva
• the .gitlab-ci.yaml shouldn't need to be modified because it already does a pip3 install diskimage-builder (which we'll still be needed) and a python3 -m pip install --user . which will install our sylva_imagebuider module

/cc @bogdan.nicolae