tests: fix regression in qa/tasks/ceph_master.py

tests: fix regression in qa/tasks/ceph_master.py

https://github.com/ceph/ceph/pull/13194 introduced a regression:

2017-02-06T16:14:23.162 INFO:tasks.thrashosds.thrasher:Traceback (most recent call last):
  File "/home/teuthworker/src/github.com_ceph_ceph_master/qa/tasks/ceph_manager.py", line 722, in wrapper
    return func(self)
  File "/home/teuthworker/src/github.com_ceph_ceph_master/qa/tasks/ceph_manager.py", line 839, in do_thrash
    self.choose_action()()
  File "/home/teuthworker/src/github.com_ceph_ceph_master/qa/tasks/ceph_manager.py", line 305, in kill_osd
    output = proc.stderr.getvalue()
AttributeError: 'NoneType' object has no attribute 'getvalue'

This is because the original patch failed to pass "stderr=StringIO()" to run().

Fixes: http://tracker.ceph.com/issues/16263
Signed-off-by: Nathan Cutler <ncutler@suse.com>
Signed-off-by: Kefu Chai <kchai@redhat.com>