Ingest tarball in test repo, and run multinode tests

[casparvl]

This describes in more detail the second part of the workflow outlined in https://github.com/EESSI/software-layer/issues/149

Proposal

1. Have a cronjob pull tarballs from S3 bucket. Ingest in a 'staging' Stratum 0 and PR e.g. EESSI/staging (to involve a reviewer)
2. Reviewer checks and approves to trigger multinode test. Which tests to run is either obtained from a file in EESSI/software-layer (e.g. #testspec.py, see example on https://github.com/EESSI/software-layer/issues/148)
3. If succesful, tarball is ingested into production Stratum 0.

Alternative 2. Which test is run is determined by the reviewer giving commands to the bot, e.g.

@bot please test @hpc_sys1 @cloud_sys1
TAGS=‘gromacs, gpu’

TODO

• [ ] Create cronjob (on the stratum 0? Or elsewhere?) to pull in tarballs
• [ ] Create CI for EESSI/staging that can connect to various HPC/cloud infra to launch multinodes test.