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/prometheus/alerta/init.yml b/prometheus/alerta/init.yml
new file mode 100644
index 0000000..a82e8d4
--- /dev/null
+++ b/prometheus/alerta/init.yml
@@ -0,0 +1,2 @@
+classes:
+  - service.prometheus.alerta