Optional deletion of all rows before Export

At the moment we have two use cases: we append to the table (no eval tool needed) or we want first to delete all the rows from the table and in the same run do the export. It's likely we will need to evaluate some other SQL queries.

It's temporary solution. I would like to refactor ExportSqoopJob and ImportSqoopJob. Those two classes have a lot of Scalding orchestration (dealing with sinks and sources, creating Flows and Riffles) which can be abstracted away. Then we could easily introduce third job DeleteAndExportSqoopJob. I'll try to explain it in an issue.