Assume that by default ZooKeeper listens on TCP/2181.

This is to improve usability a little bit by supplying default port
when such is not given i.e.

  zktop.py --servers 127.0.0.1

Using the 127.0.0.1:2181 format is no longer needed ...

Signed-off-by: Krzysztof Wilczynski <krzysztof.wilczynski@linux.com>