fix(flannel): default the interface to each host's default IPv4 interface (#689)

* fix(flannel): default the interface to each host's default IPv4 interface

- Replace the hardcoded flannel_iface: eth0 with a per-host default derived
  from ansible_facts.default_ipv4.interface
- KVM/cloud hosts that are not named eth0 (e.g. enp1s0, ens3) now resolve the
  interface automatically instead of failing the k3s_node_ip lookup
- Update the commented calico_iface / cilium_iface examples to match
- Co-authored-by: Fritz Dunkel <677609+FinalDoom@users.noreply.github.com>
- Fixes #621

* test(flannel): add regression test for per-host interface default

- Add a focused test that renders the sample inventory's flannel_iface
  expression against fake ansible facts
- Assert a non-eth0 host (enp1s0, ens3) resolves its own interface and that
  the expression defaults from ansible_facts.default_ipv4.interface
- Wire it as a local pre-commit hook (default-interface-test)

* test(molecule): assert nodes register a non-loopback InternalIP

- Add a flannel-scenario verify assertion that every node reports an
  InternalIP derived from its configured flannel_iface
- Guards against k3s binding to 127.0.0.1 instead of the cluster interface
- Complements the pre-commit default-interface test with an end-to-end check

.pre-commit-config.yaml

@@ -84,3 +84,11 @@ repos:
         language: system
         pass_filenames: false
         files: ^roles/k3s_server/tasks/metallb\.yml$|^\.github/scripts/test-metallb-remote-read\.sh$
+      - id: default-interface-test
+        name: default interface test
+        entry: python3 .github/scripts/test-default-interface.py
+        language: python
+        additional_dependencies:
+          - Jinja2>=3.1
+        pass_filenames: false
+        files: ^inventory/sample/group_vars/all\.yml$|^\.github/scripts/test-default-interface\.py$