fixes/improves weird hashing issues on readwrite tests

There is a bit of repetition in these new additions (a function and a class) but I didn't want to go ahead and refactor anything else unless it fixed the problems we were seeing in http://tracker.ceph.com/issues/10066

Adds some unit tests for the new helpers, and tested these changes against a magna box:

test yaml:
    $ cat test.yml
    readwrite:
      bucket: rwtest
      readers: 10
      writers: 3
      duration: 300
      files:
        num: 10
        size: 2000
        stddev: 500
      random_seed:
        writer: 236047946
        main: 2514311331
        names: 4153910459
        contents: 1256193726
        reader: 2312633921
    s3:
      user_id: client0-system-user
      access_key: secret
      secret_key: secret
      display_name: Mr. foo.client.0
      email: adeza@redhat.com
      host: magna077.ceph.redhat.com
      is_secure: false
      port: 8000
      port: 7280
    
And the tests:

    cat test.yml | s3tests-test-readwrite
    /home/adeza/.virtualenvs/test/local/lib/python2.7/site-packages/pkg_resources.py:991: UserWarning: /home/adeza/.python-eggs is writable by group/others and vulnerable to attack when used with get_resource_filename. Consider a more secure location (set with .set_extraction_path or the PYTHON_EGG_CACHE environment variable).
      warnings.warn(msg, UserWarning)
    Using random seeds: {'writer': 236047946, 'main': 2514311331, 'names': 4153910459, 'contents': 1256193726, 'reader': 2312633921}
    Created bucket: rwtest
    Creating random file names
    Uploading initial set of 10 files
    Starting main worker loop.
    Using file size: 2000 +- 500
    Spawning 3 writers and 10 readers...
    
    
    {bucket: rwtest, duration: 220690966, key: ztovozlqkxhqlkt, start: 1418410923.451777,
      type: r, worker: 2}
    {bucket: rwtest, duration: 89059114, key: ilzimnupha, start: 1418410923.632474, type: r,
      worker: 0}
    {bucket: rwtest, duration: 86205006, key: bsligrqsqpokbmgek, start: 1418410923.642961,
      type: r, worker: 5}
    {bucket: rwtest, duration: 285329819, key: rqelwjfddyoscw, start: 1418410923.479829,
      type: r, worker: 1}
    {bucket: rwtest, duration: 211473942, key: xsvleodvmtbggym, start: 1418410923.573156,
      type: r, worker: 8}
    {bucket: rwtest, duration: 71174145, key: yurlswmludjlot, start: 1418410923.735761,
      type: r, worker: 5}
    {bucket: rwtest, duration: 188899994, key: bsligrqsqpokbmgek, start: 1418410923.628557,
      type: r, worker: 7}
    {bucket: rwtest, duration: 1334789991, key: rqelwjfddyoscw, start: 1418410922.493062,
      type: r, worker: 9}
    {bucket: rwtest, duration: 272337914, key: yurlswmludjlot, start: 1418410923.56586,
      type: r, worker: 4}
    {bucket: rwtest, duration: 143549919, key: yurlswmludjlot, start: 1418410923.725403,
      type: r, worker: 0}
    
    .....
    {bucket: rwtest, duration: 233824968, key: xsvleodvmtbggym, start: 1418410923.675111,
      type: r, worker: 2}
    Cleaning bucket <Bucket: rwtest> key <Key: rwtest,bnnpzuaji>
    Cleaning bucket <Bucket: rwtest> key <Key: rwtest,bsligrqsqpokbmgek>
    Cleaning bucket <Bucket: rwtest> key <Key: rwtest,dsttvxnkkpw>
    Cleaning bucket <Bucket: rwtest> key <Key: rwtest,ilzimnupha>
    Cleaning bucket <Bucket: rwtest> key <Key: rwtest,lropdvmey>
    Cleaning bucket <Bucket: rwtest> key <Key: rwtest,rqelwjfddyoscw>
    Cleaning bucket <Bucket: rwtest> key <Key: rwtest,wqeclkxinxcauzatffn>
    Cleaning bucket <Bucket: rwtest> key <Key: rwtest,xsvleodvmtbggym>
    Cleaning bucket <Bucket: rwtest> key <Key: rwtest,yurlswmludjlot>
    Cleaning bucket <Bucket: rwtest> key <Key: rwtest,ztovozlqkxhqlkt>