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/global_defaults/defaults/main.yml

@@ -36,13 +36,14 @@ system_defaults:
     dns:
       servers: []
       search: []
+    interfaces: []
   path: ""
   packages: []
   disks: []
   user:
     name: ""
     password: ""
-    key: []
+    keys: []
   root:
     password: ""
   luks: