Optional deletion of all rows before Export

Actually we can't extend ExportSqoopJob, because there is no way to inherit its 4 constructors. ExportSqoopJob is in fact a factory of ExportSqoopFlow, which is in turn wrapper for ExportSqoopRiffle, which has only one interesting method 'complete'. We would need to copy&paste almost all the code. This solution meant to be quick and temporary. In exportSqoop method of Maestro/Sqoop trait we can check if sql query in SqoopOptions is set and then throw exception/log warning. 

If we don't run out of time this sprint I opt for refactoring SqoopJob-SqoopFlow-SqoopRiffle to be able to simply add few lines of interesting logic (export, import, delete+export).