Adding infrastructure to allow sending messages between adapters
using the adapter-agent event bus.

Change-Id: Ia3654b8944745bc76f8ac4ea3c1439a082979dd4
diff --git a/voltha/adapters/interface.py b/voltha/adapters/interface.py
index fcba8af..60c27f1 100644
--- a/voltha/adapters/interface.py
+++ b/voltha/adapters/interface.py
@@ -192,6 +192,17 @@
         :return: None
         """
 
+    def receive_inter_adapter_message(msg):
+        """
+        Called when the adapter recieves a message that was sent to it directly
+        from another adapter. An adapter may register for these messages by calling
+        the register_for_inter_adapter_messages() method in the adapter agent.
+        Note that it is the responsibility of the sending and receiving
+        adapters to properly encode and decode the message.
+        :param msg: The message contents.
+        :return: None
+        """
+
         # TODO work in progress
         # ...