Make it possible to run install.yml locally without root permissions

[bedroge]

Some steps now require root permissions, but in principle it should be possible to run this playbook as a regular user, e.g. by mounting some local folder as /cvmfs in the container. We should try to add some clever checks or use tags to skip certain steps to make this possible.

[bedroge]

I had a look at this, but I don't think it's (easily) possible. The prefix installation itself can be done in a different directory on the host (which is mounted as /cvmfs in the container), but for many of the other steps we would need a similar trick. For instance, doing the installation of the package/sets outside of the container will probably break things too. We could do that step in the container too, but the current Ansible portage module cannot be wrapped in a Singularity container / prefixed with a Singularity command. Same thing for, for instance, the creation of the symlinks: doing this step on the host will make incorrect symlinks, but the step cannot be run in a container.
Ultimately you would want to run almost the entire playbook in a container (using something like https://github.com/alexhrescale/singularity-ansible-plugin or https://github.com/rupertnash/ansible-singularity-plugin, though they seem to assume you're running it locally...), but then you need something else to prepare the host...

Since we want to use the automation that @terjekv created in the infrastructure repo anyway, I don't think it's worth spending a lot of time on this now.

[bedroge]

This is now implemented by running the playbook itself in a container, allowing a regular user to do the entire installation (as long as Singularity/Apptainer is available). See #167