Merge tag 'sparks-3.1.1' into develop

3.1.1
wrong conditions for custom requirement scripts.
split the requirements tasks into subtasks for custom requirements scripts parallel execution, but only ONE pip* execution per machine. Make requirements be callable alone in sparks context (eg. `fab test sdf.requirements` doesn't ask for a hostname anymore).
bump version in __init__.py
Merge branch 'release/3.1.2'