Add repository metadata

[bedroge]

Related to #67, but it may also be useful to add some custom metadata, e.g. the latest version of the repo (requested by @ocaisa).

[bedroge]

This should be added to our Stratum 0 playbook.

[bedroge]

The only tricky thing is that this playbook would need to run regularly in order to keep the latest entry up to date. Or, perhaps we could use a hook that will update this information whenever the symlink is changed:
https://cvmfs.readthedocs.io/en/stable/cpt-repo.html#customizable-actions-using-server-hooks

[rptaylor]

Not sure if the repo metadata should be used to discover the latest repo version; they are updated by separate processes.

Try enabling CVMFS_VIRTUAL_DIR on your stratum 0 and take a look in the snapshots dir:
https://cvmfs.readthedocs.io/en/stable/apx-parameters.html

This also has the advantage that users can read any revision they want without having to change the client configuration and doing a config reload. Using variant symlinks involves changing the client config (but not sure if a config reload is required in that case.)

If you really do want to lock the repository on client to a fixed catalog revision for a long period of time, use CVMFS_ROOT_HASH or CVMFS_REPOSITORY_TAG (there might be another useful parameter for that too).
But changing client config and doing cvmfs_config reload frequently to update the client is not really practical, and has the disadvantage of coupling user activity with admin privileges required to reconfigure the client.

[rptaylor]

Also keep in mind this is something you would probably want to synchronize across all nodes, and it is not possible to do an atomic update across all nodes in a cluster in a completely synchronized way, with psh or otherwise. There would always be some nodes that get changed sooner than others. So I think it would be better to handle this at the user application level, e.g. a script that selects one specific version of software to use, which will either work on all nodes or fail if the required version is not present on a node.

[rptaylor]

That being said I still recommend updating repo metainfo for other purposes, just not version discovery/management.