ipv6 subnet address mode should not only check "ipv6_address_mode" but also ipv6_ra_mode

Create an IPv6 subnet with ipv6-ra-mode is slaac or stateless, and ipv6-address-mode to none. Neutron will also allocate an stateful address to instance, which is not the instance actually get.

[root@node1 ~]# neutron subnet-show 5a2c86de-35f7-4d50-b490-9cf5f4edbe99
+-------------------+-------------------------------------------------------------------+
| Field | Value |
+-------------------+-------------------------------------------------------------------+
| allocation_pools | {"start": "2001:200::10", "end": "2001:200::ffff:ffff:ffff:fffe"} |
| | {"start": "2001:200::1", "end": "2001:200::e"} |
| cidr | 2001:200::/64 |
| dns_nameservers | |
| enable_dhcp | True |
| gateway_ip | 2001:200::f |
| host_routes | |
| id | 5a2c86de-35f7-4d50-b490-9cf5f4edbe99 |
| ip_version | 6 |
| ipv6_address_mode | |
| ipv6_ra_mode | dhcpv6-stateless |
| name | |
| network_id | 228afb74-bed4-4e66-9b5f-4bc56a37ee43 |
| tenant_id | b7843e73eea547629f26afb764fc3bef |
+-------------------+-------------------------------------------------------------------+

[root@node1 ~]# nova list
+--------------------------------------+--------------+--------+------------+-------------+----------------------------+
| ID | Name | Status | Task State | Power State | Networks |
+--------------------------------------+--------------+--------+------------+-------------+----------------------------+
| 1ef77bb5-fe7e-4f17-8fac-99e79d5abdd2 | ubuntu_less | ACTIVE | - | Running | ipv6_gre=2001:200::12 |