different check for running chef-client

Original solution for https://bugzilla.novell.com/show_bug.cgi?id=803358 fails for modules that do not import CommandLine. (It was https://github.com/yast/yast-yast2/pull/48)

So here, I'm moving the same check for running chef into earlier place: yast2 script. ContinueCancel popup is shown as well, but now started as stand-alone client.

Clients intended as running in CLI mode (= with arguments) are ignored in the check. This is consistent with original solution, although maybe that's not correct and user should be somehow warned as well.

(This is ruby version for master, for SLE I'd prepare ycp version)