Provide helper for "remove all content from a directory except these names"

[happz]

Current prune() of steps and plugins removes the whole directory. If there are members to be preserved, there's _preserved_workdir_members list, but some of the preservable members may be more dynamic, and then the custom prune() turns into a copy & paste madness. See #3511, #3548. We need a helper to do the job, then call it from these custom prune() implementations.

Consider also supporting glob-style patterns, i.e. _preserved_workdir_members = ['guest-logs/*'] should be acceptable, I see no reason why guest should list all guest logs, or why it should have its own custom prune() just to skip this directory.