ping_url has no default value

If you have a client.conf file with no ping_url value in it, its value becomes "None" and the client will never ping:

2012-09-13 15:45:55,668 ERROR [MainThread] Error contacting ping server at None
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/twisted/python/threadpool.py", line 207, in _worker
    result = context.call(ctx, function, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/twisted/python/context.py", line 118, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File "/usr/lib/python2.7/dist-packages/twisted/python/context.py", line 81, in callWithContext
    return func(*args,**kw)
  File "/usr/lib/python2.7/dist-packages/landscape/lib/fetch.py", line 98, in fetch
    raise PyCurlError(e.args[0], e.args[1])
PyCurlError: Error 6: Couldn't resolve host 'None'

It should default to hosted production, like "url" does for the message server.