rgw: handle empty conf case

rgw: handle empty conf case

2014-03-20T23:30:28.964 ERROR:teuthology.contextutil:Saw exception from nested tasks
Traceback (most recent call last):
  File "/home/teuthworker/teuthology-firefly/teuthology/contextutil.py", line 27, in nested
    vars.append(enter())
  File "/usr/lib/python2.7/contextlib.py", line 17, in __enter__
    return self.gen.next()
  File "/home/teuthworker/teuthology-firefly/teuthology/task/rgw.py", line 83, in ship_config
    idle_timeout = conf.get('idle_timeout', 30)
AttributeError: 'NoneType' object has no attribute 'get'

This happens with job yamls like:

- rgw:
  - client.0

(where it is a list and not a dict of clients)

Signed-off-by: Sage Weil <sage@inktank.com>