[prometheus] Add missing webhook for alertmanager

Signed-off-by: Yohann D'ANELLO <ynerant@crans.org>

roles/prometheus-alertmanager/templates/prometheus/alertmanager.yml.j2

@@ -56,3 +56,5 @@ receivers:
   webhook_configs:
   - url: 'http://localhost:5000/'
     send_resolved: true
+  - url: 'http://localhost:8000/'
+    send_resolved: true