Improved benchmark report formatting

- Results are displayed more compactly in the report
- Only nodes with significant performance changes are displayed in execution summary comparisons

Change-Id: I51fe1f71760ba451b23d3d0ec31358c848e54976
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3979
Reviewed-by: Taras Bobrovytsky <tbobrovytsky@cloudera.com>
Tested-by: jenkins