teuthology: s/cStringIO.StringIO/six.StringIO/

teuthology: s/cStringIO.StringIO/six.StringIO/

to be py3 compatible. this addresses the failure of
```
2020-04-05T12:35:02.036 INFO:teuthology.run_tasks:Running task ssh_keys...
2020-04-05T12:35:02.050 ERROR:teuthology.run_tasks:Saw exception from tasks.
Traceback (most recent call last):
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_py3/teuthology/run_tasks.py", line 86, in run_tasks
    manager = run_one_task(taskname, ctx=ctx, config=config)
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_py3/teuthology/run_tasks.py", line 64, in run_one_task
    task = get_task(taskname)
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_py3/teuthology/run_tasks.py", line 28, in get_task
    module = _import('tasks', module_name, task_name, fail_on_import_error=True)
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_py3/teuthology/run_tasks.py", line 52, in _import
    0,
ImportError: No module named 'tasks.ssh_keys'
```

Signed-off-by: Kefu Chai <kchai@redhat.com>