feat(network): make interfaces[] canonical, normalize flat fields as AWX compat

Flat network fields (bridge, ip, prefix, gateway, vlan) are now converted
into a single-entry interfaces[] list during normalization. All virtualization
tasks (proxmox, vmware, libvirt, xen) and configuration (NM, Alpine, Void)
now consume system_cfg.network.interfaces exclusively for multi-NIC support.

Also fixes: user.key -> user.keys in system_cfg output, strict list-only DNS
in example inventories, removes legacy single-MAC virtualization_mac_address
default.

roles/configuration/tasks/users.yml

@@ -18,7 +18,7 @@
   changed_when: configuration_user_result.rc == 0
 
 - name: Ensure .ssh directory exists
-  when: system_cfg.user.key | length > 0
+  when: system_cfg.user.keys | length > 0
   ansible.builtin.file:
     path: /mnt/home/{{ system_cfg.user.name }}/.ssh
     state: directory
@@ -27,7 +27,7 @@
     mode: "0700"
 
 - name: Add SSH public keys to authorized_keys
-  when: system_cfg.user.key | length > 0
+  when: system_cfg.user.keys | length > 0
   ansible.builtin.lineinfile:
     path: /mnt/home/{{ system_cfg.user.name }}/.ssh/authorized_keys
     line: "{{ item }}"
@@ -35,4 +35,4 @@
     group: 1000
     mode: "0600"
     create: true
-  loop: "{{ system_cfg.user.key }}"
+  loop: "{{ system_cfg.user.keys }}"