Add the slurm virtual cluster to the CI

This MR adds steps for testing the slurm schedulers in the pipeline including slurm slurm-semiglobal and slurm-openmpi.

This is done by adding a new lxd runner that sits on Maiko. The creation of the cluster and the activation of the runner is semi-automated but still needs a user to manually register the runner with gitla.inria.fr. All instructions for this process and source files are added in a new repository located here.

• setup new virtual cluster with instructions
• add new lxd runner accessible by gitlab CI
• create new CI stages employing LXD for slurm, slurm-semiglobal and slurm-openmpi
• create a virtual environment that can be reused to speed up CI stages
• create instructions for rebuilding the cluster and adding the runner
• ensure fault tolerance is tested in slurm
• add an update script to the virtual-cluster-ci repository to make updating dependencies easier than rebuilding the entire cluster.
• test new iterative stats checkpointing