storage: add support for more shared storage systems

[mdonadoni]

Currently REANA supports some shared storage systems:

• CephFS for workspaces
• EOS
• CVMFS (read only)

We need to add support for more shared storage systems, either as additional mount points or as a replacement for CephFS

• Investigate the possible different ways to mount storage systems in REANA/Kubernetes
  • Kubernetes native volumes (PV, PVC, SC, CSI drivers, ...) like CephFS
  • Mount the storage systems on each node and then propagate the mount to the jobs, like EOS
  • check out (and maybe document!) related previous work in REANA: helm: administrators to choose allowed workspace options reana#532
• Initially, add support for mounting multiple additional "scratch spaces" in run-batch/run-job pods, with a mechanism similar to EOS
  • scratch space is mounted on every node of the cluster and is then mounted inside the pods
  • objective: support GlusterFS as additional scratch space
• Bonus point: support different shared storage space for workspaces (not only CephFS)

[mdonadoni]

Note that EOS-like mounting (hostPath) can be already achieved with REANA_JOB_HOSTPATH_MOUNTS