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