Fix for Heat Salt Formula

Add correct stack_absent delete handling to prevent playing
in russian roulette. Now if stack is deleted before it's polled,
_poll_for_complete handle it correctly, without raising incorrect
exception.

Also dirty hack with sys.path.insert is removed.

Related-Issue: PROD-22102
Change-Id: I7f994c26304069052344c881b34e540e49fd5a32
3 files changed