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_libvirt_example.yml

@@ -42,7 +42,8 @@ all:
             user:
               name: "web"
               password: "CHANGE_ME"
-              key: "ssh-ed25519 AAAA..."
+              keys:
+                - "ssh-ed25519 AAAA..."
             root:
               password: "CHANGE_ME"
             packages:
@@ -83,7 +84,8 @@ all:
             user:
               name: "db"
               password: "CHANGE_ME"
-              key: "ssh-ed25519 AAAA..."
+              keys:
+                - "ssh-ed25519 AAAA..."
             root:
               password: "CHANGE_ME"
             luks:
@@ -111,7 +113,9 @@ all:
               prefix: 24
               gateway: 192.168.122.1
               dns:
-                servers: "1.1.1.1,1.0.0.1"
+                servers:
+                  - "1.1.1.1"
+                  - "1.0.0.1"
             disks:
               - size: 80
               - size: 200
@@ -121,7 +125,8 @@ all:
             user:
               name: "compute"
               password: "CHANGE_ME"
-              key: "ssh-ed25519 AAAA..."
+              keys:
+                - "ssh-ed25519 AAAA..."
             root:
               password: "CHANGE_ME"
             features: