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

inventory/sample/group_vars/all.yml

@@ -8,16 +8,18 @@ systemd_dir: /etc/systemd/system
 system_timezone: Your/Timezone
 
 # interface which will be used for flannel
-flannel_iface: eth0
+# Defaults to each host's default IPv4 interface (e.g. eth0, enp1s0, ens3)
+# so KVM/cloud hosts without eth0 work out of the box. Override per-host if needed.
+flannel_iface: "{{ ansible_facts.default_ipv4.interface }}"
 
 # uncomment calico_iface to use tigera operator/calico cni instead of flannel https://docs.tigera.io/calico/latest/about
-# calico_iface: "eth0"
+# calico_iface: "{{ ansible_facts.default_ipv4.interface }}"
 calico_ebpf: false # use eBPF dataplane instead of iptables
 calico_tag: v3.32.1 # calico version tag
 
 # uncomment cilium_iface to use cilium cni instead of flannel or calico
 # ensure v4.19.57, v5.1.16, v5.2.0 or more recent kernel
-# cilium_iface: "eth0"
+# cilium_iface: "{{ ansible_facts.default_ipv4.interface }}"
 cilium_mode: native # native when nodes are on the same subnet or use BGP, otherwise set tunnel
 cilium_tag: v1.20.0 # cilium version tag
 cilium_cli_tag: v0.19.7 # cilium cli version tag