fix: replica set deployment for multi tenants (#8673)

Changes in IP underneath are dynamic in replica sets
with multiple tenants, so deploying in that fashion
will not work until we wait for atleast one participatory
server to be local.

This PR also ensures that multi-tenant zone expansion also
works in replica set k8s deployments.

Introduces a new ENV `KUBERNETES_REPLICA_SET` check to call
appropriate code paths.

cmd/notification.go

@@ -798,6 +798,10 @@ func (sys *NotificationSys) RemoveRulesMap(bucketName string, rulesMap event.Rul
 
 // ConfiguredTargetIDs - returns list of configured target id's
 func (sys *NotificationSys) ConfiguredTargetIDs() []event.TargetID {
+	if sys == nil {
+		return nil
+	}
+
 	sys.RLock()
 	defer sys.RUnlock()