Arvados

Reviewing 3505-virtual-work-dir at 02d6330. Looks pretty good overall, just a few small comments:

• Would it make sense to organize these Crunch script utility modules, like robust_put and vwd, under a subdirectory of crunch_scripts? I feel like it would be nice if people could ls crunch_scripts and see at a glance a list of the scripts that ship with Arvados. Tucking the non-runnable modules into a subdirectory would make that easier.
• robust_put's exception handlers use the logging module, but it isn't imported.
• As a general rule, var is None is more natural Python style than var == None.

Thanks.

5f62b0c looks good to merge. Thanks.