[metallb] Add variable for the metallb namespace (#11136)

roles/kubernetes-apps/metallb/templates/layer2.yaml.j2

@@ -11,7 +11,7 @@ apiVersion: metallb.io/v1beta1
 kind: L2Advertisement
 metadata:
   name: "{{ entry }}"
-  namespace: metallb-system
+  namespace: "{{ metallb_namespace }}"
 spec:
   ipAddressPools:
   - "{{ entry }}"