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
1 file changed