scaleway: Fix failing terraform test

[hakman]

/home/prow/go/src/k8s.io/kops/tests/integration/update_cluster/minimal_scaleway
 �[31m╷
�[31m│ �[31mError: Unsupported attribute
�[31m│ 
�[31m│   on kubernetes.tf line 243, in resource "scaleway_lb_backend" "lb-backend-https":
�[31m│  243:   server_ips       = [scaleway_instance_server.control-plane-fr-par-1-0.private_ip]
�[31m│ 
�[31m│ This object has no argument, nested block, or exported attribute named
�[31m│ "private_ip". Did you mean "private_ips"?
�[31m╵
�[31m╷
�[31m│ �[31mError: Unsupported attribute
�[31m│ 
�[31m│   on kubernetes.tf line 252, in resource "scaleway_lb_backend" "lb-backend-kops-controller":
�[31m│  252:   server_ips       = [scaleway_instance_server.control-plane-fr-par-1-0.private_ip]
�[31m│ 
�[31m│ This object has no argument, nested block, or exported attribute named
�[31m│ "private_ip". Did you mean "private_ips"?
�[31m╵

[hakman]

/cc @rifelpet @ameukam @upodroid

[hakman]

/approve

[upodroid]

LGTM

[k8s-ci-robot]

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