Summary

• Add create_ipv6_secondary_networks.yml prepare task that creates IPv6 networks
  (slaac + dhcpstateless), subnets, and router for secondary worker interfaces
• Add configure_ipv6_secondary day2ops procedure that applies DHCP kernel arg
  MachineConfig, patches CNO with macvlan additionalNetworks, deploys test pods,
  and verifies pod-to-pod IPv6 connectivity
• Fix naming mismatch: validation now reads secondary_ip_protocol instead of
  non-existent network_secondary
• Add 4.18_ipi-ipv6-secondary.yaml job definition for the scenario
• Add IPv6 secondary network config block to configs/global.yml

Details

Prepare phase:

• Creates 2 IPv6 networks with different RA modes (slaac, dhcpv6-stateless)
• Creates IPv6 subnets with appropriate RA/address modes
• Creates a router and attaches both subnets

Post-install (day2ops procedure):

• Applies 05-worker-kernelarg-dhcp MachineConfig for ip=dhcp,dhcp6
• Waits for MCP update and cluster health
• Discovers IPv6 interfaces on workers
• Patches CNO network.operator with macvlan additionalNetworks
• Deploys hello-openshift pods with IPv6 network annotations
• Verifies pod-to-pod connectivity on both slaac and dhcpstateless networks
• Validates external reachability via oc debug node/ from worker nodes

Future Scope (not in this PR)

• IPv6 VM connectivity testing: Deploy a standalone OpenStack VM attached to the
  IPv6 secondary networks to validate full VM-to-pod and pod-to-VM connectivity.
  Deferred because it requires admin-only OpenStack operations (image upload to Glance,
  flavor creation) which are not available from the shiftstack (non-admin) cloud.
  IR plugin reference: roles/day2ops/tasks/install_ipv6_vm.yml (205 lines),
  roles/day2ops/tasks/check_ipv6_connectivity.yml (100 lines).
• Worker scaleup re-verification: After scaling up workers, re-scale IPv6 deployments
  and re-check connectivity. IR plugin reference:
  roles/day2ops/tasks/scale-ipv6-networks-deployments.yml (66 lines).

Test Plan

• ./gate.sh passes (ansible-lint + pre-commit)
• IPv6 secondary networks created in OpenStack
• Workers have IPv6 interfaces after MachineConfig application
• NetworkAttachmentDefinition CRs created by CNO
• Pods can communicate over IPv6 secondary interfaces (same node + different nodes)
• External reachability validated via oc debug node/