add configuration for eessi.io

[boegel]

After a lot of preparatory work, we now finally have a new CernVM-FS repository in place for the European Environment for Scientific Software Installations (EESSI), which is part of the effort to make EESSI ready for production that is funded via the MultiXscale EuroHPC Centre-of-Excellence.

The contents of this repository will gradually be expanded over the coming weeks/months/years via the semi-automated contribution workflow we have implemented and documented.
Later this year we would like to announce this repository as being the production-ready setup of EESSI.

It would be great to have this configuration be part of the default CernVM-FS configuration repository, especially since we will be using EESSI as a running example in the upcoming "Best Practices for CernVM-FS in HPC" tutorial.

cc @bedroge, @ocaisa, @casparvl, @trz42

[DrDaveD]

I suggest changing the key name to eessi.io.pub, so it implies that it applies to all repositories of the eessi.io domain. I know there's only one so far but there may be more in the future.

[boegel]

@DrDaveD If that's not required, I think we prefer to keep it as is for now, to leave the door open to use a different key per repository (unless that doesn't make much sense).

I guess we can easily change software.eessi.io.pub to eessi.io.pub later, or would that be a hassle?

Getting the configuration for software.eessi.io repository added is important for the upcoming tutorial, so we would prefer if that can be done quickly.

[DrDaveD]

This looks good to me. It's up to @jblomer now to review, merge, and deploy.

After he has approved I'll also want a cherry-pick PR to the master branch. For egi and osg it probably requires a request from somebody in one of the supported VOs in those projects.