Manual task trigger should reset outputs to "not completed".

Yi Xiao reported that if ```foo => bar``` has already succeeded, then she manually triggers ```foo``` again, and then sets ```bar``` to the waiting state, ```bar``` will trigger immediately (i.e. before the retriggered ```foo``` is finished).  The problem is that manual triggering was not resetting ```foo```'s outputs to "not completed".

@matthewrmshin - please review.