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_example.yml

@@ -25,6 +25,7 @@ system:
   memory: 8192
   balloon: 0
   network:
+    # Flat fields (AWX survey compatibility, builds single-entry interfaces[])
     bridge: "vmbr0"
     ip: "{{ inventory_hostname }}"
     prefix: 24
@@ -35,6 +36,16 @@ system:
         - "1.0.0.1"
       search:
         - "example.com"
+    # Multi-NIC: use interfaces[] instead of flat fields above
+    # interfaces:
+    #   - name: "eth0"
+    #     bridge: "vmbr0"
+    #     ip: "10.0.0.10"
+    #     prefix: 24
+    #     gateway: "10.0.0.1"
+    #   - name: "eth1"
+    #     bridge: "vmbr1"
+    #     vlan: "100"
   path: "/Lab/Example"
   disks:
     - size: 80
@@ -47,7 +58,8 @@ system:
   user:
     name: "ops"
     password: "CHANGE_ME"
-    key: "ssh-ed25519 AAAA..."
+    keys:
+      - "ssh-ed25519 AAAA..."
   root:
     password: "CHANGE_ME"
   luks: