Add alerta UI for prometheus alerts

This change add new service with:
* The new service in monitoring stack: Alerta UI
* Nginx proxy entity for Alerta UI
* Relabeling for prometheus alerts
* New webhook receiver

Change-Id: Id042f2fa04a5881cd48265fa6e7a327237024acc
Prod-related: PROD-19597
diff --git a/nginx/server/proxy/monitoring/alerta.yml b/nginx/server/proxy/monitoring/alerta.yml
new file mode 100644
index 0000000..432a9dc
--- /dev/null
+++ b/nginx/server/proxy/monitoring/alerta.yml
@@ -0,0 +1,18 @@
+parameters:
+  nginx:
+    server:
+      enabled: true
+      site:
+        nginx_proxy_alerta:
+          enabled: true
+          type: nginx_proxy
+          name: alerta
+          proxy:
+            host: ${_param:prometheus_control_address}
+            port: 15017
+            protocol: http
+          host:
+            name: ${_param:cluster_public_host}
+            port: 15017
+            protocol: https
+          ssl: ${_param:nginx_proxy_ssl}