usage example:
```
gh-ost ... --force-table-names=migration_tbl
```
The force-table-names flag will receive a table name and use that name
when creating the _del, _gho, _ghc tables.
For instance for that example above, the tables would be:
_migration_tbl_del
_migration_tbl_gho
_migration_tbl_ghc
[fixes#399]
843c74f960