Add worker environment propagation mode

[Kobzol]

WIP, doesn't have tests or documentation tests. Also I'm not sure if this makes more sense on the worker or the task (or both?).

Related issue: #1038

[spirali]

LGTM

I have the following commets:

• Propagation policy should be a property of a task (because of our philosophy that worker is a "provider" and task a "consumer" who chooses what it needs from a worker).
• I would add third option "StripManager" that strips all SLURM_ and PBS_ variables
• Question is what should be default. I would vote for "isolate" or "strip manager" as it is less surprising option than "propage", despite both are breaking changes (StripManager would probably break only a few codes).