Introduce $CLUSTERSHELL_CFGDIR

[kcgthb]

Introduce $CLUSTERSHELL_CFGDIR as an alternate location for configuration files.

This is basically a revisit of #191. This original issue led to #111, which allows per-user configuration file overrides, but doesn't provide a way to define system-wide configuration files (not per-user), which are not stored in /etc.

The use case is a multi-node cluster, where ClusterShell could be provided as a user-facing tool, installed on a shared file system (like a NFS mount point), and not as a local package on each node. In that case, providing system defaults for all users would requiring distributing configuration files on each and every of the cluster node's local /etc/ directory. A more flexible approach would be to define an environment variable (the proposed $CLUSTERSHELL_CFGDIR) to point to the global configuration directory, which could be co-located with the shared ClusterShell installation.

When $CLUSTERSHELL_CFGDIR is not defined, the default /etc/clustershell is used as a fallback to keep compatibility with existing configurations.

This PR:

• updates lib/ClusterShell/Defaults.py
• updates (text) documentation files in doc/
• adds a test in tests/CLIConfigTest.py

[degremont]

Just ensure the new variable name matches other possible variables we already support, if they exist. (CLUSTERSHELL_ prefix).

[kcgthb]

Just ensure the new variable name matches other possible variables we already support, if they exist. (CLUSTERSHELL_ prefix).

Yes. The only other environment variables I could find are the CLUSTERSHELL_GW_* env vars, so we should be god.
https://github.com/cea-hpc/clustershell/blob/master/lib/ClusterShell/Worker/Tree.py#L179-L182