Fix incorrect subnet_id for ipv6 member servers

When adding an ipv6 member in test_ipv6_traffic_ops, the ipv4 subnet
from the same network was provided. This is incorrect, the ipv6 subnet
must be passed.

Change-Id: Id8f409a15e14c110f8075bbe943aed42224a948d
diff --git a/octavia_tempest_plugin/tests/scenario/v2/test_ipv6_traffic_ops.py b/octavia_tempest_plugin/tests/scenario/v2/test_ipv6_traffic_ops.py
index 4efa241..69c1f2b 100644
--- a/octavia_tempest_plugin/tests/scenario/v2/test_ipv6_traffic_ops.py
+++ b/octavia_tempest_plugin/tests/scenario/v2/test_ipv6_traffic_ops.py
@@ -431,8 +431,9 @@
             const.ADDRESS: self.webserver1_ipv6,
             const.PROTOCOL_PORT: 80,
         }
-        if self.lb_member_1_subnet:
-            member1_kwargs[const.SUBNET_ID] = self.lb_member_1_subnet[const.ID]
+        if self.lb_member_1_ipv6_subnet:
+            member1_kwargs[const.SUBNET_ID] = (
+                self.lb_member_1_ipv6_subnet[const.ID])
 
         member1 = self.mem_member_client.create_member(
             **member1_kwargs)
@@ -455,8 +456,9 @@
             const.ADDRESS: self.webserver2_ipv6,
             const.PROTOCOL_PORT: 80,
         }
-        if self.lb_member_2_subnet:
-            member2_kwargs[const.SUBNET_ID] = self.lb_member_2_subnet[const.ID]
+        if self.lb_member_2_ipv6_subnet:
+            member2_kwargs[const.SUBNET_ID] = (
+                self.lb_member_2_ipv6_subnet[const.ID])
 
         member2 = self.mem_member_client.create_member(**member2_kwargs)
         self.addCleanup(