As in #2892, I think it would be useful for automation to support the 'uuid-file' argument in any situation where a pod UUID is expected as an argument.

So far as I can tell, that is currently the case (but not yet supported) for:

• cat-manifest
• enter
• run-prepared
• status

Also, 'rkt prepare' should accept the '--uuid-file-save' argument.

I believe this would make a couple of things easier when running rkt pods from systemd where you can't easily use shell substitutions. For instance, I could use ExecReload=rkt enter --uuid-file=somefile apachectl graceful to reload an apache running in a rkt pod whereas now I have to kludgily use rkt prepare, save the outputted uuid to a file to be used as an EnvironmentFile and then pass that as an argument when reloading. (If someone has a better solution for that btw I'd love to hear it.)