Updates to perfdiag.

 * Track which 5xx codes were returned, rather than just the count.
 * Check whether HTTPS was enabled during the test, as it effects performance.
 * Change get_contents_to_string calls during download tests to use
   get_contents_to_file with a dummy file-like object. Downloading to a string is
   inefficient because it needlessly accumulates the downloaded data with StringIO.
   The dummy file just throws away the bytes, which gets much better performance.
 * Add boto and gsutil versions to JSON output.