qa/tasks/cephfs: allow caller to use BytesIO when calling rados()

qa/tasks/cephfs: allow caller to use BytesIO when calling rados()

when the caller expects binary data, it should pass BytesIO as stdout.

this change shold address the failure of
```
2020-04-05T12:47:25.335 INFO:tasks.cephfs_test_runner:======================================================================
2020-04-05T12:47:25.336 INFO:tasks.cephfs_test_runner:ERROR: test_apply_tag (tasks.cephfs.test_forward_scrub.TestForwardScrub)
2020-04-05T12:47:25.336 INFO:tasks.cephfs_test_runner:----------------------------------------------------------------------
2020-04-05T12:47:25.336 INFO:tasks.cephfs_test_runner:Traceback (most recent call last):
2020-04-05T12:47:25.337 INFO:tasks.cephfs_test_runner:  File "/home/teuthworker/src/github.com_tchaikov_ceph_wip-qa-py3/qa/tasks/cephfs/test_forward_scrub.py", line 75, in test_apply_tag
2020-04-05T12:47:25.337 INFO:tasks.cephfs_test_runner:    self.assertTagged(inos[dirpath], tag, self.fs.get_metadata_pool_name())
2020-04-05T12:47:25.337 INFO:tasks.cephfs_test_runner:  File "/home/teuthworker/src/github.com_tchaikov_ceph_wip-qa-py3/qa/tasks/cephfs/test_forward_scrub.py", line 98, in assertTagged
2020-04-05T12:47:25.338 INFO:tasks.cephfs_test_runner:    "scrub_tag"
2020-04-05T12:47:25.338 INFO:tasks.cephfs_test_runner:  File "/home/teuthworker/src/github.com_tchaikov_ceph_wip-qa-py3/qa/tasks/cephfs/test_forward_scrub.py", line 35, in _read_str_xattr
2020-04-05T12:47:25.339 INFO:tasks.cephfs_test_runner:    strlen = struct.unpack('i', output[0:4])[0]
2020-04-05T12:47:25.339 INFO:tasks.cephfs_test_runner:TypeError: a bytes-like object is required, not 'str'
```

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