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.

main.yml

@@ -44,9 +44,9 @@
                     'name': (
                       (system_user_input.name | default('') | string | length) > 0
                     ) | ternary(system_user_input.name | string, prompt_user_name),
-                    'key': (
-                      system_user_input.key
-                      if (system_user_input.key is iterable and system_user_input.key is not string and system_user_input.key | length > 0)
+                    'keys': (
+                      system_user_input.keys
+                      if (system_user_input.keys is iterable and system_user_input.keys is not string and system_user_input.keys | length > 0)
                       else (
                         [prompt_user_key]
                         if (prompt_user_key | length > 0)