task/ceph_manager:  Fix NoneType config issue

kill_mon is getting a config set to None, which blows
up now due to the check for powercycle.  Initialize
the config to an empty dict if we don't get anything
on init.  This is the error showing up in teuthology:

2013-02-04T15:04:16.595 ERROR:teuthology.run_tasks:Manager failed: <contextlib.GeneratorContextManager object at 0x1fcafd0>
Traceback (most recent call last):
  File "/var/lib/teuthworker/teuthology-master/teuthology/run_tasks.py", line 45, in run_tasks
    suppress = manager.__exit__(*exc_info)
  File "/usr/lib/python2.7/contextlib.py", line 24, in __exit__
    self.gen.next()
  File "/var/lib/teuthworker/teuthology-master/teuthology/task/mon_thrash.py", line 142, in task
    thrash_proc.do_join()
  File "/var/lib/teuthworker/teuthology-master/teuthology/task/mon_thrash.py", line 69, in do_join
    self.thread.get()
  File "/var/lib/teuthworker/teuthology-master/virtualenv/local/lib/python2.7/site-packages/gevent/greenlet.py", line 308, in get
    raise self._exception
AttributeError: 'NoneType' object has no attribute 'get'

Signed-off-by: Sam Lang <sam.lang@inktank.com>