Pass in Alerta authentication details for Alertmanager to send alerts with

Related-PROD: PROD-35884
Change-Id: I5ee52fb3fd81a48464414102d909baaeb4a1c18d
diff --git a/prometheus/alerta/init.yml b/prometheus/alerta/init.yml
index 6e16b63..2e82f44 100644
--- a/prometheus/alerta/init.yml
+++ b/prometheus/alerta/init.yml
@@ -1,5 +1,5 @@
 classes:
-  - service.prometheus.alerta
+- service.prometheus.alerta
 parameters:
   prometheus:
     alerta:
diff --git a/prometheus/alertmanager/notification/alerta.yml b/prometheus/alertmanager/notification/alerta.yml
index 816aaf6..2ab3509 100644
--- a/prometheus/alertmanager/notification/alerta.yml
+++ b/prometheus/alertmanager/notification/alerta.yml
@@ -18,3 +18,7 @@
               alerta:
                 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}"