Fix Network object encoding issue when using qpid

When creating network via REST API will throw the exception
"CodecException: no encoding for
<nova.db.sqlalchemy.models.Network object at 0x4a8ea10>".
It only occurs when using qpid.
Fix this issue through using jsonutils.to_primitive method.

Fix bug 1143584.

Change-Id: I833aef451f8e3d5eaa246c19d67b2474a9d36910
(cherry picked from commit b4d43b0)

nova/network/manager.py

@@ -1646,7 +1646,7 @@ def find_next(subnet):
 
             if network and cidr and subnet_v4:
                 self._create_fixed_ips(context, network['id'], fixed_cidr)
-        return networks
+        return jsonutils.to_primitive(networks)
 
     @wrap_check_policy
     def delete_network(self, context, fixed_range, uuid,