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.

inventory_example.yml

@@ -45,7 +45,8 @@ all:
             user:
               name: "ops"
               password: "CHANGE_ME"
-              key: "ssh-ed25519 AAAA..."
+              keys:
+                - "ssh-ed25519 AAAA..."
             root:
               password: "CHANGE_ME"
             packages:
@@ -89,7 +90,9 @@ all:
               prefix: 24
               gateway: 10.0.0.1
               dns:
-                servers: "1.1.1.1,1.0.0.1"
+                servers:
+                  - "1.1.1.1"
+                  - "1.0.0.1"
             disks:
               - size: 80
               - size: 200
@@ -99,7 +102,8 @@ all:
             user:
               name: "dbadmin"
               password: "CHANGE_ME"
-              key: "ssh-ed25519 AAAA..."
+              keys:
+                - "ssh-ed25519 AAAA..."
             root:
               password: "CHANGE_ME"
             luks: