Optional deletion of all rows before Export

Hrmm, this seems like potentially quite surprising behavior to an unsuspecting user who happened to set an sql query on the options for whatever reason. I know there's no particular reason for a user to be doing that, but there's nothing stopping him from doing so either. How much work would it be to create a `DeleteAndExportSqoopJob` that extended from `ExportSqoopJob`, and avoid potentially surprising our users?

BTW, the `Job` classes will be replaced by the `Execution` work we want to roll out over the next sprint or two (the `Flow` classes will stay though).