Adding support for OLT side metadata field

Due to historic reasons, ONOS injects a metadata match field into one
of the downstream unicats flow rules, which was not yet handled by voltha.
That is fixed and tested now.

Change-Id: Ic8a47de515fa5837a70941be22da9e2d6539f614
diff --git a/ponsim/ponsim.py b/ponsim/ponsim.py
index c855e39..3086ff5 100644
--- a/ponsim/ponsim.py
+++ b/ponsim/ponsim.py
@@ -138,9 +138,12 @@
                     return False
 
             elif field.type == UDP_DST:
-                if field.udsp_dst != get_udp_dst(frame):
+                if field.udp_dst != get_udp_dst(frame):
                     return False
 
+            elif field.type == METADATA:
+                pass  # safe to ignore
+
             else:
                 raise NotImplementedError('field.type=%d' % field.type)