Update Aggregator default preview config

Author: jpraynaud

mithril-aggregator/config/preview.json

@@ -11,7 +11,8 @@
     "url_snapshot_manifest": "https://storage.googleapis.com/cardano-preview/snapshots.json",
     "snapshot_store_type": "local",
     "snapshot_uploader_type": "gcp",
+    "snapshot_bucket_name": "cardano-testnet",
     "stores_directory": "./mithril-aggregator/stores",
     "genesis_verification_key": "5b3132372c37332c3132342c3136312c362c3133372c3133312c3231332c3230372c3131372c3139382c38352c3137362c3139392c3136322c3234312c36382c3132332c3131392c3134352c31332c3233322c3234332c34392c3232392c322c3234392c3230352c3230352c33392c3233352c34345d",
     "limit_keys_in_stores": 5
-}
+}