With the removal of node.max_local_storage_nodes, there is no need anymore to keep the data in
subfolders indexed by a node ordinal. This commit makes it so that ES 8.0 will store data directly in
$DATA_DIR instead of $DATA_DIR/nodes/$nodeOrdinal.
Upon startup, Elasticsearch will check to see if there is data in the old location, and automatically
move it to the new location. This automatic migration only works if $nodeOrdinal is 0, i.e., multiple
node instances have not previously run on the same data path, which required for
node.max_local_storage_nodes to explicitly be configured.
Yannick Welsch
Vladimir Dolzhenko