U/salnikov/dm 621

Most of these operation aren't required for mysql data loading, so an cli option for plain mysql data loading should be available in order to use the loader in integration test for plain mysql data loading. Please note that having a different code for MySQL data loading seems reasonable (this code is simple and allow to check DataLoader._loadData(). Indeed if the same loader is used for Qserv and MySQL, integration tests will not test it.