Add test_level_placement and fix it with the 2.2.0+ PG "folding" system

mon/osd_tree.js

@@ -87,7 +87,7 @@ function make_hier_tree(global_config, tree)
     tree[''] = { children: [] };
     for (const node_id in tree)
     {
-        if (node_id === '' || !(tree[node_id].children||[]).length && (tree[node_id].size||0) <= 0)
+        if (node_id === '')
         {
             continue;
         }

mon/pg_gen.js

@@ -178,7 +178,7 @@ async function generate_pool_pgs(state, global_config, pool_id, osd_tree, levels
     const rules = use_rules ? get_pg_rules(pool_id, pool_cfg, global_config.placement_levels) : null;
     const folded = fold_failure_domains(Object.values(pool_tree), use_rules ? rules : [ [ [ pool_cfg.failure_domain ] ] ]);
     // FIXME: Remove/merge make_hier_tree() step somewhere, however it's needed to remove empty nodes
-    const folded_tree = make_hier_tree(global_config, folded.nodes);
+    const folded_tree = make_hier_tree(global_config, folded.nodes.reduce((a, c) => { a[c.id] = c; return a; }, {}));
     const old_pg_count = prev_pgs.length;
     const optimize_cfg = {
         osd_weights: folded.nodes.reduce((a, c) => { if (Number(c.id)) { a[c.id] = c.size; } return a; }, {}),