PostgreSQL driver should not be hard-coded to public schema

#### What are you trying to achieve?
In src/Codeception/Lib/Driver/PostgreSql.php, the schema is hard-coded to "public" in method cleanup. In my setup, where the schema used for testing is not public, this causes tests to fail with a ModuleException:
` [ModuleException] Db: SQLSTATE[42501]: Insufficient privilege: 7 ERROR: must be owner of schema public`
The schema is taken into account in "load" method, and this should also be brought into cleaning up the test database.

### Details

* Codeception version: 2.3.7
* PHP Version: 7.0.22
* Operating System: Ubuntu
* Installation type: Phar
* List of installed packages (`composer show`)

src/Codeception/Lib/Driver/PostgreSql.php から始め、load メソッドのスキーマ処理と cleanup を比較します。public 以外のテスト用スキーマを使用して失敗を再現し、cleanup が public スキーマの所有権をもはや必要としないことを確認します。

索引モデルが issue の本文から書いたものです。