Add possiblity to set CLUSTER_ID via env var

- mitigate missing alert_relabel_configs in Cortex
- if missing in the alert labels it will be added

Change-Id: I89bcc0672aa663178d02e69bdc941c1fcc63db80
Related-PROD: PRODX-24796
diff --git a/sf_notifier/server.py b/sf_notifier/server.py
index 8077627..3702992 100644
--- a/sf_notifier/server.py
+++ b/sf_notifier/server.py
@@ -56,7 +56,9 @@
         try:
             alert['labels']['env_id'] = sf_cli.environment
             fields, action = alert_fields_and_action(
-                alert, add_links=settings.ADD_LINKS)
+                alert,
+                add_links=settings.ADD_LINKS,
+                cluster_id=settings.CLUSTER_ID)
         except KeyError as key:
             msg = 'Alert misses {} key.'.format(key)
             app.logger.error(msg)