Use mapped storage for Atomix management group in tests to preserve consistency across releases

Change-Id: I17ef2e120ec275ec03f6a21359b12abfd5981eaf
diff --git a/tools/test/bin/atomix-gen-config b/tools/test/bin/atomix-gen-config
index f42d152..c3c89ac 100755
--- a/tools/test/bin/atomix-gen-config
+++ b/tools/test/bin/atomix-gen-config
@@ -135,7 +135,10 @@
           'type': 'raft',
           'partitions': 1,
           'partitionSize': len(nodes),
-          'members': [node['id'] for node in nodes]
+          'members': [node['id'] for node in nodes],
+          'storage': {
+              'level': 'mapped'
+          }
       },
       'partitionGroups': {
           'raft': {