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/settings/production.py b/sf_notifier/settings/production.py
index 6251daf..6458662 100644
--- a/sf_notifier/settings/production.py
+++ b/sf_notifier/settings/production.py
@@ -40,6 +40,7 @@
 }
 
 ADD_LINKS = is_true(os.environ.get('SF_NOTIFIER_ADD_LINKS'))
+CLUSTER_ID = os.environ.get('SF_NOTIFIER_CLUSTER_ID')
 
 SF_CONFIG = {
     # Salesforce login params