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/cli/main.py b/cli/main.py
index fa778a1..12a157f 100755
--- a/cli/main.py
+++ b/cli/main.py
@@ -498,7 +498,8 @@
                 priority=500,
                 match_fields=[
                     in_port(nni_port_no),
-                    vlan_vid(4096 + 1000)
+                    vlan_vid(4096 + 1000),
+                    metadata(40)  # here to mimic an ONOS artifact
                 ],
                 actions=[pop_vlan()],
                 next_table_id=1