[stable-only] Set IPv6 params to fix tempest-slow-py3

The job has been fixed in tempest [1] but stein is pinned to tempest
26.0.0 so any user of tempest-slow-py3 needs to apply the change
as neutron did [2].

[1] https://review.opendev.org/c/openstack/tempest/+/774764
[2] https://review.opendev.org/c/openstack/neutron/+/777389

Related-Bug: #1914037
Change-Id: I068cf1e9618d305b5a9383c283bfa0f120bfe905

.zuul.yaml

@@ -61,6 +61,13 @@
             irrelevant-files: *gate-irrelevant-files
         - tempest-slow-py3:
             irrelevant-files: *gate-irrelevant-files
+            vars:
+              devstack_localrc:
+                # to avoid https://bugs.launchpad.net/neutron/+bug/1914037
+                # as we couldn't backport the fix to stein release
+                IPV6_PUBLIC_RANGE: 2001:db8:0:10::/64
+                IPV6_PUBLIC_NETWORK_GATEWAY: 2001:db8:0:10::2
+                IPV6_ROUTER_GW_IP: 2001:db8:0:10::1
         - tempest-full:
             irrelevant-files: *gate-irrelevant-files
         - tempest-full-py3: