Variables related to DDL are not auto adjusting

## Enhancement

The variables related to DDL are not being adjusted on the size of the machine (CPU cores, etc)

```
sql> SHOW VARIABLES LIKE 'tidb_ddl%';
+----------------------------+--------------+
| Variable_name | Value |
+----------------------------+--------------+
| tidb_ddl_error_count_limit | 512 |
| tidb_ddl_reorg_batch_size | 256 |
| tidb_ddl_reorg_priority | PRIORITY_LOW |
| tidb_ddl_reorg_worker_cnt | 4 |
+----------------------------+--------------+
4 rows in set (0.0017 sec)
```

This means that manual tuning is needed to get better DDL performance