juju doesn't honor proxy-url in kube config

[Fernando Marino]

my .kube/config file has the following content:

```yaml
apiVersion: v1
clusters:
- cluster:
    certificate-authority-data: LS0.....
    server: https://10.xxx.xxx.xxx:6443
    proxy-url: http://moi.esa.int:33128
  name: default
```

juju doesn't honor the proxy-url line and force me to work around it by using the env variable proxy_http, which may pose problems elsewhere. The proxy shall be used only to access the cluster by juju as specified from the kube/config file.