Code review comment for lp://staging/~brendan-donegan/checkbox-certification/bug1185026

Lines 145-159 of the diff remove the ability to submit a file via stdin. The file-to-submit argument is now effectively mandatory, but this is not expressed in the arg parsing section, with the result that, if I call the script without a submission filename, I get a rather cryptic error:

$ bin/checkbox-certification-submit --hwid a00D000000Kkk5j
2013-05-28 11:56:27,057 INFO Preparing to submit results using Hardware ID: a00D000000Kkk5j
2013-05-28 11:56:27,058 INFO Preparing to upload submission via REST to https://certification.canonical.com/submissions/submit/
Traceback (most recent call last):
  File "bin/checkbox-certification-submit", line 153, in <module>
    sys.exit(main())
  File "bin/checkbox-certification-submit", line 125, in main
    with open(args.results_file, "rb") as submission_file:
TypeError: invalid file: None

This should be fixed somehow. I'm not sure the ability to pipe stdin to the script was used much, so the easiest way is to just make args.results_file mandatory, but I'm happy with other possible solutions.