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/cloud_playbooks/roles/packet-ci/vars/main.yml

@@ -0,0 +1,12 @@
+---
+_vm_count_dict:
+  separate: 3
+  separate-scale: 3
+  ha: 3
+  ha-scale: 3
+  ha-recover: 3
+  ha-recover-noquorum: 3
+  all-in-one: 1
+  default: 2
+
+vm_count: "{{ _vm_count_dict[mode | d('default')] }}"