mailman: Always restart mailman.service

It has been killed by systemd-oomd a few times recently and we don't
want to start it manually every time it happens.

roles/mailman/files/override.conf

+[Service]
+Restart=always

roles/mailman/handlers/main.yml

 ---
+- name: restart mailman
+  service: name=mailman daemon_reload=yes state=restarted
+
 - name: reload mailman
   service: name=mailman state=reloaded
 

roles/mailman/tasks/main.yml

@@ -48,6 +48,13 @@
 - name: start and enable postfix
   systemd: name=postfix.service enabled=yes daemon_reload=yes state=started
 
+- name: create drop-in directory for mailman.service
+  file: path=/etc/systemd/system/mailman.service.d state=directory owner=root group=root mode=0755
+
+- name: install drop-in for mailman.service
+  copy: src=override.conf dest=/etc/systemd/system/mailman.service.d/ owner=root group=root mode=0644
+  notify: restart mailman
+
 - name: start and enable mailman{.service,-*.timer}
   systemd: name={{ item }} enabled=yes daemon_reload=yes state=started
   loop: