Implemented saving drill output files to persistence.

1. Added a property DRILL_OUTPUT_DIR in the drillTestConfig.properties file to specify where to save output files.  This directory is directly under "resources".  If it is not set, /tmp will be used instead.
2. Output file names are constructed in the following way: DRILL_OUTPUT_DIR/testId_queryName.output_<current_date_time>.
3. Minor refactoring.