Add dry-run-like option

[casparvl]

EESSI CLI abstracts away a lot of complexity, e.g. of CVMFS. This is nice - until things don't work, or you want to do something slightly different.

Since EESSI CLI essentially just runs certain sequences of commands (e.g. eessi check would in the background just invoke several cvmfs commands), it would be nice to just be able to dump those commands - to stdout, or to a script. That ensures that the CLI tool doesn't become a 'black box', and also provides more flexibility: if something doesn't quire work on your system, you can use the CLI tool to get the full list of commands, and maybe change one option.

[boegel]

I'm very much in favor of avoiding that this becomes a black box, but it's not just running the commands, it also "parsing" the output to extract certain fields, etc.

Maybe having a --debug option which shows which commands are being run, and maybe even which regex patterns are used to extract info, is good enough?