Fix virtual_machines variable declaration syntax

Remove redundant outer object wrapper in virtual_machines declaration
that was causing Terraform configuration error.

terraform/prod/terraform.tfvars

@@ -33,7 +33,7 @@ admin_ssh_public_key_path = "~/.ssh/keys/id_rsa.pub"
 # Utilisateur par défaut sur les VMs
 default_vm_user = "damien"
 
-virtual_machines = { virtual_machines = {
+virtual_machines = {
   "ipfabric" = {
     os      = "ipfabric"
     cores   = 4
@@ -43,7 +43,7 @@ virtual_machines = { virtual_machines = {
     gateway = "192.168.1.254"
     tags    = ["ipfabric"]
   }
-} }
+}
 
 #############################
 # Notes de configuration