adds stateful ruler clause for boltdb shipper jsonnet (grafana#3297)

production/ksonnet/loki/boltdb_shipper.libsonnet

@@ -10,6 +10,7 @@
     // run ingesters and queriers as statefulsets when using boltdb-shipper to avoid using node disk for storing the index.
     stateful_ingesters: if self.using_boltdb_shipper then true else super.stateful_ingesters,
     stateful_queriers: if self.using_boltdb_shipper then true else super.stateful_queriers,
+    stateful_rulers: if self.using_boltdb_shipper then true else super.stateful_rulers,
 
     boltdb_shipper_shared_store: error 'must define boltdb_shipper_shared_store when using_boltdb_shipper=true. If this is not intentional, consider disabling it. boltdb_shipper_shared_store is a backend key from the storage_config, such as (gcs) or (s3)',
     compactor_pvc_size: '10Gi',