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.

vars_baremetal_example.yml

@@ -28,7 +28,8 @@ system:
   user:
     name: "admin"
     password: "CHANGE_ME"
-    key: "ssh-ed25519 AAAA..."
+    keys:
+      - "ssh-ed25519 AAAA..."
   root:
     password: "CHANGE_ME"
   luks: