Use memory mapped file storage for Raft partitions in tests

Change-Id: I68c3e4befcde059711c6df1b094b578200c00c62
diff --git a/tools/test/bin/atomix-gen-config b/tools/test/bin/atomix-gen-config
index e69f441..f42d152 100755
--- a/tools/test/bin/atomix-gen-config
+++ b/tools/test/bin/atomix-gen-config
@@ -142,7 +142,10 @@
               'type': 'raft',
               'partitions': num_partitions,
               'partitionSize': partition_size,
-              'members': [node['id'] for node in nodes]
+              'members': [node['id'] for node in nodes],
+              'storage': {
+                  'level': 'mapped'
+              }
           }
       }
   }