fix(ci_gen_kustomize_values): add MTU to uni07eta net-attach-def JSON

[fyanac]

The uni07eta network-values template already emitted mtu on each network YAML block but the embedded Multus net-attach-def JSON omitted it, so ctlplane and other NADs ignored cifmw_networking_definition.mtu (e.g. jumbo for ospbr / ctlplane).

Include "mtu": {{ network.mtu | default(1500) }} in the ctlplane, octavia, ironic, and generic macvlan net-attach-def.

Assisted-By: Cursor-composer-2-fast

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