[stable/2023.2] Fix `libvirt_exporter` missing `namespaceSelector: (#1529)

This is an automated cherry-pick of #1517
/assign mnaser
diff --git a/roles/kube_prometheus_stack/vars/main.yml b/roles/kube_prometheus_stack/vars/main.yml
index 4a3d030..277c937 100644
--- a/roles/kube_prometheus_stack/vars/main.yml
+++ b/roles/kube_prometheus_stack/vars/main.yml
@@ -448,6 +448,9 @@
             relabelings: *relabelings_instance_to_node_name
       - name: libvirt-exporter
         jobLabel: job
+        namespaceSelector:
+          matchNames:
+            - openstack
         selector:
           matchLabels:
             application: libvirt