gitlab_runner: enable free-page-reporting

https://lore.kernel.org/linux-mm/20200211224416.29318.44077.stgit@localhost.localdomain/

roles/gitlab_runner/templates/domain_template.xml.j2

@@ -37,6 +37,7 @@
       <model type='virtio'/>
       <filterref filter='clean-traffic'/>
     </interface>
+    <memballoon model='virtio' freePageReporting='on'/>
     <rng model='virtio'>
       <backend model='random'>/dev/urandom</backend>
     </rng>