Make sure output from extra args is strings

Setting the following:

```
kube_kubeadm_controller_extra_args:
  address: 0.0.0.0
  terminated-pod-gc-threshold: "100"
```

Results in `terminated-pod-gc-threshold: 100` in the kubeadm config file. But it has to be a string to work.

roles/kubernetes/master/templates/kubeadm-config.yaml.j2

@@ -59,7 +59,7 @@ apiServerExtraArgs:
 {% endif %}
   allow-privileged: "true"
 {% for key in kube_kubeadm_apiserver_extra_args %}
-  {{ key }}: {{ kube_kubeadm_apiserver_extra_args[key] }}
+  {{ key }}: {{ kube_kubeadm_apiserver_extra_args[key]|string }}
 {% endfor %}
 controllerManagerExtraArgs:
   node-monitor-grace-period: {{ kube_controller_node_monitor_grace_period }}
@@ -69,12 +69,12 @@ controllerManagerExtraArgs:
   feature-gates: {{ kube_feature_gates|join(',') }}
 {% endif %}
 {% for key in kube_kubeadm_controller_extra_args %}
-  {{ key }}: {{ kube_kubeadm_controller_extra_args[key] }}
+  {{ key }}: {{ kube_kubeadm_controller_extra_args[key]|string }}
 {% endfor %}
 {% if kube_kubeadm_scheduler_extra_args|length > 0 %}
 schedulerExtraArgs:
 {% for key in kube_kubeadm_scheduler_extra_args %}
-  {{ key }}: {{ kube_kubeadm_scheduler_extra_args[key] }}
+  {{ key }}: {{ kube_kubeadm_scheduler_extra_args[key]|string }}
 {% endfor %}
 {% endif %}
 apiServerCertSANs: