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/docker/swarm/stack/monitoring/alerta.yml b/docker/swarm/stack/monitoring/alerta.yml
new file mode 100644
index 0000000..c0da7d5
--- /dev/null
+++ b/docker/swarm/stack/monitoring/alerta.yml
@@ -0,0 +1,30 @@
+classes:
+- system.prometheus.alerta
+parameters:
+  _param:
+    docker_image_alerta: docker-prod-local.artifactory.mirantis.com/mirantis/external/alerta-web:latest
+    alerta_mongodb_uri: "mongodb://${_param:cluster_node01_address}:27017,${_param:cluster_node02_address}:27017,${_param:cluster_node03_address}:27017/alerta?replicaSet=stacklight"
+    alerta_admin_username: "admin@alerta.io"
+  docker:
+    client:
+      stack:
+        monitoring:
+          service:
+            alerta:
+              networks:
+                - monitoring
+              deploy:
+                labels:
+                  com.mirantis.monitoring: "alerta"
+                restart_policy:
+                  condition: any
+              labels:
+                com.mirantis.monitoring: "alerta"
+              image: ${_param:docker_image_alerta}
+              ports:
+                - 15017:8080
+              volumes:
+                - ${prometheus:alerta:config_dir}/alerta.conf:/web/config.js
+              environment:
+                ADMIN_USERS: ${_param:alerta_admin_username}
+                MONGO_URI: ${_param:alerta_mongodb_uri}
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