Keep DNS Nameserver Order Consistency In Neutron

一个subnet有多个dns server时，dns server在创建时就定好了，但可以update：

neutron subnet-update 1a2d261b-b233-3ab9-902e-88576a82afa6 
--dns_nameservers list=true 3.3.3.3 2.2.2.2 1.1.1.1

changzhi@stack:~/devstack$ neutron subnet-show 1a2d261b-b233-3ab9-902e-88576a82afa6
+------------------+--------------------------------------------+
| Field            | Value                                      |
+------------------+--------------------------------------------+
| allocation_pools | {"start": "10.0.0.2", "end": "10.0.0.254"} |
| cidr             | 10.0.0.0/24                                |
| dns_nameservers  | 3.3.3.3                                    |
|                  | 2.2.2.2                                    |
|                  | 1.1.1.1                                    |
| enable_dhcp      | True                                       |
| gateway_ip       | 10.0.0.1                                   |
| host_routes      |                                            |
| id               | 1a2d26fb-b733-4ab3-992e-88554a87afa6       |
| ip_version       | 4                                          |
| name             |                                            |
| network_id       | a404518c-800d-2353-9193-57dbb42ac5ee       |
| tenant_id        | 3868290ab10f417390acbb754160dbb2           |
+------------------+--------------------------------------------+

http://docs.openstack.org/developer/neutron/devref/dns_order.html