decdn_node role: expose node_to_node_pull_through_enabled as a knob

[alpergundogdu]

Summary

The daemon's cache.node_to_node_pull_through_enabled setting gates all serve-time origin pull-through, but the decdn_node role does not expose it — there is no decdn_* variable and node.toml.j2 never emits the key. Operators who need it must hand-edit /etc/decdn/node.toml on the host, which the next make deploy (re-templating node.toml) silently reverts.

Proposed fix

• Add a role default decdn_node_to_node_pull_through_enabled: false in roles/decdn_node/defaults/main.yml.
• Emit it in roles/decdn_node/templates/node.toml.j2 under [cache] (e.g. node_to_node_pull_through_enabled = {{ decdn_node_to_node_pull_through_enabled | lower }}), only when the operator opts in, matching the existing [cache] field conventions.

Notes

• decdn_cache_origin_path is already a role knob (defaults/main.yml); this issue is specifically about the missing pull-through toggle.
• Encountered while getting a kind: fs origin to serve a client fetch; the toggle had to be added by hand to node.toml and did not survive as inventory-managed config.