Ok, that didn't work, try this.

roles/taskotron/buildslave-configure/tasks/main.yml

@@ -49,7 +49,7 @@
   sudo_user: '{{ item.user }}'
   command: creates={{ item.dir }} buildslave create-slave {{ item.dir }} {{ buildmaster }} buildslave passwd
   with_items:
-    - '{{ slaves|default['','',''] }}'
+    - '{{ slaves|default('') }}'
   when: deployment_type == 'dev'
 
 - name: generate slave config