Use explict 'attr_dict' when adding provider->consumer edge

Instead of using the less explicit **kwarg support when adding
an edge between a explicit producer and consumer use the 'attr_dict'
keyword argument instead and use the constant defined the the flow
module as the key into that dictionary (this also ensure that the
key will be adjusted automatically if that key value ever changes).

Change-Id: Ieeae83b984b7797320997c0c4cb4289eb1a837ee

taskflow/engines/action_engine/compiler.py

@@ -33,6 +33,7 @@ _RETRY_EDGE_DATA = {
     flow.LINK_RETRY: True,
 }
 _EDGE_INVARIANTS = (flow.LINK_INVARIANT, flow.LINK_MANUAL, flow.LINK_RETRY)
+_EDGE_REASONS = flow.LINK_REASONS
 
 
 class Compilation(object):
@@ -155,11 +156,15 @@ class Linker(object):
                         "Non-invariant link being created from '%s' ->"
                         " '%s' even though the target '%s' was found to be"
                         " decomposed into an empty graph" % (v, u, u))
-                for provider in u_g:
-                    for consumer in v_g:
-                        reasons = provider.provides & consumer.requires
-                        if reasons:
-                            graph.add_edge(provider, consumer, reasons=reasons)
+                for u in u_g.nodes_iter():
+                    for v in v_g.nodes_iter():
+                        depends_on = u.provides & v.requires
+                        if depends_on:
+                            _add_update_edges(graph,
+                                              [u], [v],
+                                              attr_dict={
+                                                  _EDGE_REASONS: depends_on,
+                                              })
             else:
                 # Connect nodes with no predecessors in v to nodes with no
                 # successors in the *first* non-empty predecessor of v (thus