Consider optional manual jobs in --run-manual-jobs

We missed this one from the original MR, and I think this is a valid point in some cases. /cc @dolwup @benjamb

The following discussion from !226 (closed) should be addressed:

• @marge-triage-bot started a discussion:

  Created by: sereinity

              if job['status'] == 'manual' and job['allow_failure'] == False: