Read device serial_number as soon as the device is activated

Change-Id: Idedfb2060fb174c068a26e24ae982b18ffc51226
diff --git a/xos/synchronizer/steps/sync_olt_device.py b/xos/synchronizer/steps/sync_olt_device.py
index 1140173..4646dbd 100644
--- a/xos/synchronizer/steps/sync_olt_device.py
+++ b/xos/synchronizer/steps/sync_olt_device.py
@@ -116,6 +116,7 @@
 
         model.admin_state = request['admin_state']
         model.oper_status = request['oper_status']
+        model.serial_number = request['serial_number']
 
         if model.oper_status != "ACTIVE":
             raise Exception("It was not possible to activate OLTDevice with id %s" % model.id)