Changes for production mode allowing customization of work directory, output director...

Changes for production mode allowing customization of work directory, output directory and other variables.
Few other changes in AliAnalysisGrid interface:
- Users should now use plugin->SetTerminateFiles("file1.root,file2.root") for the files produced in their task Terminate.
- Merging in one stage can be requested explicitely: plugin->SetOneStageMerging(kTRUE) to avoid default behavior.
- Name of the validation script can be customized: plugin->SetValidationScript()