Update alerta receiver for alertmanager with basic http auth

Related-Prod: PROD-36816
Change-Id: I3f1240316151dc9255a1f5ae04618706f4f68eb8
diff --git a/prometheus/alertmanager/notification/alerta.yml b/prometheus/alertmanager/notification/alerta.yml
index 729aa26..bedf76d 100644
--- a/prometheus/alertmanager/notification/alerta.yml
+++ b/prometheus/alertmanager/notification/alerta.yml
@@ -16,5 +16,10 @@
             enabled: true
             webhook_configs:
               alerta:
-                url: "http://${_param:alertmanager_notification_alerta_host}:${_param:alertmanager_notification_alerta_port}/api/webhooks/prometheus?api-key=${_param:alerta_admin_key}"
+                url: "http://${_param:alertmanager_notification_alerta_host}:${_param:alertmanager_notification_alerta_port}/api/webhooks/prometheus"
                 send_resolved: true
+                http_config:
+                  basic_auth:
+                    username: ${_param:alerta_admin_username}
+                    password: ${_param:alerta_admin_password}
+