Make reading of the ZooKeeper configuration file more resilient.

This is to fix the following problem:

  Traceback (most recent call last):
    File "zktop.py", line 351, in <module>
      ui = Main(get_zk_servers(options.configfile))
    File "zktop.py", line 341, in get_zk_servers
      config = read_zk_config(options.configfile)
    File "zktop.py", line 337, in read_zk_config
      return config
  UnboundLocalError: local variable 'config' referenced before assignment

Signed-off-by: Krzysztof Wilczynski <krzysztof.wilczynski@linux.com>