trivial: Add orientation='reverse' to find_cycle in layering

This patchset adds orientation='reverse' when looking for
substitution cycles in the layering module in order to work
with newer versions of networkx which throw:

networkx.exception.NetworkXUnfeasible: Graph contains a cycle.

This corrects the issue.

Change-Id: I854a10ed524daf45bef9ad856938ffedb3e76baf

deckhand/engine/layering.py

@@ -337,7 +337,7 @@ class DocumentLayering(object):
                     g.add_edge(document.meta, src.meta)
 
         try:
-            cycle = find_cycle(g)
+            cycle = find_cycle(g, orientation='reverse')
         except networkx.exception.NetworkXNoCycle:
             pass
         else: