[dev.eessi.io] enable `eessi_container.sh` to mount several CVMFS repos with individual access mode

[trz42]

This is a prerequisite for dev.eessi.io.

Currently, eessi_container.sh mounts two CVMFS repositories:

• /cvmfs/cvmfs-config.cern.ch
• SOME software repository provided via --repository [default: /cvmfs/software.eessi.io]

If we want to support building for other repositories particularly on top of SOME, the script needs to be changed

• to allow and mount multiple CVMFS repositories instead of just one
• to enable access modes to be individually set for each repository (rw - for read & write, ro - for read-only)

For each CVMFS repository to be mounted, the script should manage sets of parameters and temporary directories. It should also store these settings somewhere such that we can easily resume with the same configuration.

The format for the value of --repository could be changed by supporting a suffix such as ,access=ACCESS_MODE to specify the access mode for a repository. May want to add another suffix to provide repository-specific host_injections (,host_injections=HOST_PATH:CONTAINER_PATH).

The script sources other scripts of EESSI/software-layer, e.g., init/eessi_defaults. The need and way to source these scripts may need to be revisited (are they needed, do they need to be changed too, etc).