Add taskflow (used by cinder + others) to requirements

Taskflow (a stackforge based library) targeted at helping
openstack projects with state management and consistency
is now offically a library and to remove the cinder local
copy we need to add it as a requirement.

Change-Id: Ic112d707b3d914481da41adbb6aa9d8b890742df
Merge "Add taskflow (used by cinder + others) to requirements"