Namespaces : Onyxia should accept existing namespaces for deployments

In https://github.com/InseeFrLab/onyxia-api/blob/master/onyxia-api/src/main/java/fr/insee/onyxia/api/services/impl/kubernetes/KubernetesService.java

The API is expecting to have created the namespace here :
```java
public List getNamespaces(Region region, Owner owner) {
KubernetesClient kubClient = kubernetesClientProvider.getRootClient(region);
return kubClient.namespaces().withLabel("onyxia_owner",owner.getId()).list().getItems();
}
```

Should it be different?