Fix the IPv6 tests for no ipv6-public-subnet

The IPv6 tests will fail if the ipv6-public-subnet (created by the
tempest plugin) is not present. This is because we don't store
the lb_member_vip_ipv6_net value when we create an ipv6 subnet.

Change-Id: Ib44896c0707f29cb6b98bb249176602f3e7c7190
diff --git a/octavia_tempest_plugin/tests/test_base.py b/octavia_tempest_plugin/tests/test_base.py
index 5ade75e..6389c6b 100644
--- a/octavia_tempest_plugin/tests/test_base.py
+++ b/octavia_tempest_plugin/tests/test_base.py
@@ -314,6 +314,7 @@
                     'ip_version': 6}
                 result = cls.lb_mem_subnet_client.create_subnet(
                     **subnet_kwargs)
+                cls.lb_member_vip_ipv6_net = cls.lb_member_vip_net
                 cls.lb_member_vip_ipv6_subnet = result['subnet']
                 cls.addClassResourceCleanup(
                     waiters.wait_for_not_found,