Adding serial_number information to OLTDevice

Change-Id: Ied682803092f0538cf5beaf59dcaeedd65be14c5
diff --git a/xos/synchronizer/pull_steps/pull_olts.py b/xos/synchronizer/pull_steps/pull_olts.py
index 0f3c199..6c57cde 100644
--- a/xos/synchronizer/pull_steps/pull_olts.py
+++ b/xos/synchronizer/pull_steps/pull_olts.py
@@ -129,6 +129,7 @@
             model.device_id = olt["id"]
             model.admin_state = olt["admin_state"]
             model.oper_status = olt["oper_status"]
+            model.serial_number = olt['serial_number']
 
             model.volt_service = self.volt_service
             model.volt_service_id = self.volt_service.id