osd: don't return list reference across pushy magic

This avoids a very confusing error like

Traceback (most recent call last):
  File "./ceph-deploy", line 9, in <module>
    load_entry_point('ceph-deploy==0.1', 'console_scripts', 'ceph-deploy')()
  File "/home/ubuntu/cdep/ceph-deploy/ceph_deploy/cli.py", line 112, in main
    return args.func(args)
  File "/home/ubuntu/cdep/ceph-deploy/ceph_deploy/osd.py", line 428, in osd
    prepare(args, cfg, activate_prepared_disk=True)
  File "/home/ubuntu/cdep/ceph-deploy/ceph_deploy/osd.py", line 273, in prepare
    s = '{} returned {}\n{}\n{}'.format(cmd, ret, out, err)
  File "/home/ubuntu/cdep/ceph-deploy/virtualenv/local/lib/python2.7/site-packages/pushy-0.5.1-py2.7.egg/pushy/protocol/proxy.py", line 271, in <lambda>
    setattr(ProxyClass, "__str__", lambda self: conn.getstr(self))
  File "/home/ubuntu/cdep/ceph-deploy/virtualenv/local/lib/python2.7/site-packages/pushy-0.5.1-py2.7.egg/pushy/protocol/connection.py", line 78, in getstr
    return self.send_request(MessageType.getstr, object)
  File "/home/ubuntu/cdep/ceph-deploy/virtualenv/local/lib/python2.7/site-packages/pushy-0.5.1-py2.7.egg/pushy/protocol/baseconnection.py", line 296, in send_request
    raise Exception, "Connection is closed"
Exception: Connection is closed

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