Copy rt_list before modifying

For transit networks, we had the following code:
    rt_list = vn.rt_list
    rt_list |= set([vn.get_route_target()])

Because of assigning the vn.rt_list to rt_list, and then using, '|=' operator,
we unintentionally ended up modifying vn.rt_list. We should make a copy
of vn.rt_list before modifying.

Change-Id: I1b4b8eac86a47992923681a0f29918248240e8cb
Closes-Bug: 1499922

src/config/schema-transformer/to_bgp.py

@@ -1818,9 +1818,9 @@ def _create(self, si_info):
 
             if first_node:
                 first_node = False
-                rt_list = vn1_obj.rt_list
+                rt_list = set(vn1_obj.rt_list)
                 if vn1_obj.allow_transit:
-                    rt_list |= set([vn1_obj.get_route_target()])
+                    rt_list.add(vn1_obj.get_route_target())
                 service_ri1.update_route_target_list(rt_list,
                                                      import_export='export')
 
@@ -1854,9 +1854,9 @@ def _create(self, si_info):
             _vnc_lib.routing_instance_update(service_ri1.obj)
             _vnc_lib.routing_instance_update(service_ri2.obj)
 
-        rt_list = vn2_obj.rt_list
+        rt_list = set(vn2_obj.rt_list)
         if vn2_obj.allow_transit:
-            rt_list |= set([vn2_obj.get_route_target()])
+            rt_list.add(vn2_obj.get_route_target())
         service_ri2.update_route_target_list(rt_list, import_export='export')
 
         service_ri2.add_connection(vn2_obj.get_primary_routing_instance())