Allow configuration of verbose and trace modes

 - verbose option adds -v flag to cats run
 - sets up artifact_directory so that test artifacts are saved to the
   /var/vcap/sys/log/acceptance_tests directory
 - rename .txt files to .log files so that bosh run errand
   --download-logs is able to pull down cf_trace logs

Signed-off-by: Michael Fraenkel <fraenkel@us.ibm.com>