Adding serial_number information to OLTDevice

Change-Id: Ied682803092f0538cf5beaf59dcaeedd65be14c5
diff --git a/xos/synchronizer/steps/sync_olt_device.py b/xos/synchronizer/steps/sync_olt_device.py
index 301f123..1140173 100644
--- a/xos/synchronizer/steps/sync_olt_device.py
+++ b/xos/synchronizer/steps/sync_olt_device.py
@@ -84,7 +84,9 @@
             raise Exception(
                 'VOLTHA Device Id is empty. This probably means that the OLT device is already provisioned in VOLTHA')
         else:
-            model.device_id = res['id'];
+            model.device_id = res['id']
+            model.serial_number = res['serial_number']
+
 
         return model