Avoid duplication of code in all the "base singleuser" images

[garciagenrique]

• vre-singleuser
• vre-singleuser-py38
• vre-singleuser-root

are all base singleuser images from which other user images are later derived.

--> Create a subdir with the common bin and configuration files common for all these images to avoid maintaining 3 times the same setup.sh, configuration.py and kernel.json files.

However, this would most likely need the modification of the CI file.

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