Avoid conflicts with OS python packages by using Python venv in /opt/rucio

[bziemons]

Closes: #458

[bziemons]

Please mind this needs manual testing. I might have missed something

[rcarpa]

I like, a lot, the idea of having a venv for rucio rather than using the global python libraries, but I wonder if mixing rucio binaries and the venv ones is a good idea. I would personally put the venv in a different location

[Geogouz]

Right now, the baked venv at /opt/rucio is basically hidden by the bind mounts we use in the docker-compose. So we should adjust this on rucio/rucio side too. But we could maybe also use a venv like /opt/rucio-venv, set the PATH to that venv and PYTHONPATH=/opt/rucio/lib for the module resolution.

Also, another idea might be to break this into separate PR per container. It feels like reviewing all of them at once, comes with the disadvantage of having a huge scope. Which means that this will take quite a long time to be merged. I think we should at least start with the dev container and see how this goes. That one is not very dangerous to merge and play with. Dunno.

[rcarpa]

I like the idea of trying it on the dev container first. Then do the others.