ceph_manager: use get() for self.config powercycle checks

ceph_manager: use get() for self.config powercycle checks

I think this is what is going on...

Traceback (most recent call last):
  File "/var/lib/teuthworker/teuthology-master/teuthology/contextutil.py", line 27, in nested
    yield vars
  File "/var/lib/teuthworker/teuthology-master/teuthology/task/ceph.py", line 1158, in task
    yield
  File "/var/lib/teuthworker/teuthology-master/teuthology/run_tasks.py", line 25, in run_tasks
    manager = _run_one_task(taskname, ctx=ctx, config=config)
  File "/var/lib/teuthworker/teuthology-master/teuthology/run_tasks.py", line 14, in _run_one_task
    return fn(**kwargs)
  File "/var/lib/teuthworker/teuthology-master/teuthology/task/dump_stuck.py", line 93, in task
    manager.kill_osd(id_)
  File "/var/lib/teuthworker/teuthology-master/teuthology/task/ceph_manager.py", line 665, in kill_osd
    if 'powercycle' in self.config and self.config['powercycle']:
TypeError: argument of type 'NoneType' is not iterable