Fixed test_runner.sh argument order.

The README file indicates the following parameter order for executing the JMeter tests:

./test_runner.sh {groupname} {descriptioname} testusers.csv testplan.jmx site_data.properties

But, after failures and examining the test_runner.sh script, I realised that the parameter order should be:

./test_runner.sh {groupname} {descriptioname} testplan.jmx testusers.csv site_data.properties