feat(neutron-understack): Add/remove gateway: trunk-based VIF attach and cleanup PUC-1920

[nidzrai]

Add/remove gateway: trunk-based VIF attach and cleanup

What does this change do?

Upgrade impact

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[cardoe]

Code review

Found 1 issue:

1. Leftover PR-specific image tag left in place for neutron_server (this pattern was flagged as needing to be reverted before merge on a previous PR, feat: PUC-1758: remove physical_network fallback lookup #2145, which had the same issue)

neutron_server is pointed at ghcr.io/rackerlabs/understack/neutron:pr-2256 instead of the pinned release tag 2026.1. This looks like a leftover from spinning up this PR's own e2e-test image build rather than an intentional change, and would leave configs pointing at an ephemeral, non-reproducible PR image if merged as-is.

understack/components/images-openstack.yaml

Lines 44 to 48 in 232ed2e

	neutron_ovn_metadata: "ghcr.io/rackerlabs/understack/neutron:2026.1"
	neutron_openvswitch_agent: "ghcr.io/rackerlabs/understack/neutron:2026.1"
	neutron_server: "ghcr.io/rackerlabs/understack/neutron:pr-2256"
	neutron_rpc_server: "ghcr.io/rackerlabs/understack/neutron:2026.1"
	neutron_bagpipe_bgp: "ghcr.io/rackerlabs/understack/neutron:2026.1"

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