Open Stack

There is a change up to add taskflow to the global requirements. I have
no problem with this in principle, but it's one more that's in the set
of things like pecan, wsme and friends that are in the set of things
that Sean talked about in preemptively gate the universe.

I'd like to not add it until we have a plan for at least assymetrical
gating, so that changes to taskflow at least can't break cinder and friends.

Further, I think we might need to discuss how to include libraries such
as this. Should taskflow be a part of oslo?