CI: streamline packet jobs definition

- Take advantage of `parallel:matrix` to make the jobs definition shorter
  and more readable.
- Remove helper scripts which are no longer needed
- Remove redundant indirection in the gitlab-ci pipelines definitions
  (only one user)

tests/files/debian12-custom-cni-helm.yml

@@ -0,0 +1,22 @@
+---
+# Instance settings
+cloud_image: debian-12
+
+# Kubespray settings
+kube_owner: root
+kube_network_plugin: custom_cni
+custom_cni_chart_namespace: kube-system
+custom_cni_chart_release_name: cilium
+custom_cni_chart_repository_name: cilium
+custom_cni_chart_repository_url: https://helm.cilium.io
+custom_cni_chart_ref: cilium/cilium
+custom_cni_chart_version: 1.16.3
+custom_cni_chart_values:
+  cluster:
+    name: kubespray
+  hubble:
+    enabled: false
+  ipam:
+    operator:
+      clusterPoolIPv4PodCIDRList:
+        - "{{ kube_pods_subnet }}"