revert changing decentralizationParam to 0.1

We need to keep this one quite high at the moment in order to prevent the cluster from rolling back too often. The wallet can handle rollback quite nicely, but the integration scenarios cannot (most of them). A lot of scenarios are built following a basic fixture -> action -> assertions principle, and rolling back the fixture can have very negative effects on the rest. We could run every scenario multiple times (at least twice) to maybe cope with intermediate failures due to rollback...

lib/shelley/test/data/cardano-node-shelley/genesis.yaml

@@ -6,7 +6,7 @@ protocolParams:
   protocolVersion:
     minor: 0
     major: 0
-  decentralisationParam: 0.1 # means 90% decentralised
+  decentralisationParam: 0.97 # means 3% decentralised
   maxTxSize: 4096
   minFeeA: 100
   maxBlockBodySize: 239857