Support networkx 2.0
Support networkx 2.0

Networkx 2.0 released recently.  The main difference for us is that
"node" is no longer a dictionary and should be accessed via "nodes",
and the topological_sort returns an interator

Closes-Bug: 1712693
Change-Id: I78e89f2261b8b8d28c68b517c1e61691ab40016c

openstack/diskimage-builder,switch-ch/diskimage-builder,openstack/diskimage-builder,switch-ch/diskimage-builder