s3ci.py matched the wrong job, then fellover looking for the build number

[Curtis Hovey]

As seen at
    http://reports.vapour.ws/releases/issue/58e6df12749a566238ddf08e

s3ci.py was looking for build=-centos, but that and build-centos-1-8. It fell over looking for the build number because it assumed the job name was an exact match.

Traceback (most recent call last):
  File "/var/lib/jenkins/juju-ci-tools/s3ci.py", line 209, in <module>
    sys.exit(main())
  File "/var/lib/jenkins/juju-ci-tools/s3ci.py", line 157, in main
    return cmd_get(args)
  File "/var/lib/jenkins/juju-ci-tools/s3ci.py", line 178, in cmd_get
    args.file_pattern, args.workspace):
  File "/var/lib/jenkins/juju-ci-tools/s3ci.py", line 140, in fetch_files
    file_keys = find_file_keys(bucket, revision_build, job, file_pattern)
  File "/var/lib/jenkins/juju-ci-tools/s3ci.py", line 124, in find_file_keys
    build = int(match.group(1))
AttributeError: 'NoneType' object has no attribute 'group'