Fix pushkin-vip URL for alertmanager notification settings

'haproxy_pushkin_bind_host' that is used for the parameter
'alertmanager_notification_pushkin_host' contains a host name,
it is not an URL.

Change-Id: I0bb36f2da7a78df79048875c4014262f30efe620
Closes-Bug:#PROD-15832
diff --git a/prometheus/alertmanager/notification/pushkin.yml b/prometheus/alertmanager/notification/pushkin.yml
index 905a292..9d27a8b 100644
--- a/prometheus/alertmanager/notification/pushkin.yml
+++ b/prometheus/alertmanager/notification/pushkin.yml
@@ -18,5 +18,5 @@
             enabled: true
             webhook_configs:
               pushkin-vip:
-                url: "${_param:alertmanager_notification_pushkin_host}:${_param:alertmanager_notification_pushkin_port}/webhook?app=stacklight"
+                url: "http://${_param:alertmanager_notification_pushkin_host}:${_param:alertmanager_notification_pushkin_port}/webhook?app=stacklight"
                 send_resolved: true