Support EESSI

[JohnGarbutt]

Add support for https://eessi.github.io/docs/, providing additional lmod modules.

This will be enabled in the "fat" image and in the everything template provided by the cookiecutter template.

Note deployments of more than a handful of nodes should consider providing and configuring a squid proxy for EESSI.

Install is based off this script:
https://github.com/EESSI/eessi-demo/blob/main/scripts/install_cvmfs_eessi_RHEL.sh

[sjpb]

The .sh this is based on seems to be the same as the native installation instructions, which says:

☝ The commands above only cover the basic installation of EESSI.
This is good enough for an individual client, or for testing purposes, but for a production-quality setup you should also set up a Squid proxy cache.
For large-scale systems, like an HPC cluster, you should also consider setting up your own CernVM-FS Stratum-1 mirror server.

Squid is generally probably a useful thing to deploy anyway ...

We could instead use their singularity container too:

The easiest way to access the EESSI pilot repository is by using Singularity. If Singularity is installed already, no admin privileges are required. No other software is needed either on the host.

[sjpb]

Role needs a README.md, that's all.

[sjpb]

TODO:

• needs fat image build running
• needs fat image testing in CI

[sjpb]

Fat image build WIP: https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/4863217900/jobs/8670617599

Didn't have a refreshed image build

[sjpb]

Fat image build: https://github.com/stackhpc/ansible-slurm-appliance/actions/runs/4870492195

ETA: has built openhpc-230503-0944-bf8c3f63.qcow2