CI: Document the 'all-in-one' layout + small refactoring (#10725)

* Rename aio to all-in-one and document it

ADTM.
Acronyms don't tell much.

* Refactor vm_count in tests provisioning

tests/files/packet_ubuntu22-calico-all-in-one.yml

@@ -0,0 +1,27 @@
+---
+# Instance settings
+cloud_image: ubuntu-2204
+mode: all-in-one
+vm_memory: 1600Mi
+
+# Kubespray settings
+auto_renew_certificates: true
+
+# Currently ipvs not available on KVM: https://packages.ubuntu.com/search?suite=focal&arch=amd64&mode=exactfilename&searchon=contents&keywords=ip_vs_sh.ko
+kube_proxy_mode: iptables
+enable_nodelocaldns: False
+
+containerd_registries:
+  "docker.io": "https://mirror.gcr.io"
+
+containerd_registries_mirrors:
+  - prefix: docker.io
+    mirrors:
+      - host: https://mirror.gcr.io
+        capabilities: ["pull", "resolve"]
+        skip_verify: false
+  - prefix: 172.19.16.11:5000
+    mirrors:
+      - host: http://172.19.16.11:5000
+        capabilities: ["pull", "resolve", "push"]
+        skip_verify: true