zkfarmer: end ZK connection cleanly at exit

When exiting, close the connection to ZooKeeper. Otherwise, we may not
have stdout anymore and logging messages will raise an exception. An
easy what to show the problem is to use:

    zkfarmer get /services/dbackup/10.35.2.3 backuplast | cat

It is likely that we get this exception:

    1357639290
    Exception in thread Thread-3 (most likely raised during interpreter shutdown
    Traceback (most recent call last):
      File "/usr/lib/python2.6/threading.py", line 532, in __bootstrap_inner
      File "/usr/lib/python2.6/threading.py", line 484, in run
      File "/usr/lib/pymodules/python2.6/kazoo/protocol/connection.py", line 390
      File "/usr/lib/pymodules/python2.6/kazoo/protocol/connection.py", line 472
      File "/usr/lib/python2.6/socket.py", line 188, in close
    <type 'exceptions.TypeError'>: 'NoneType' object is not callable
Merge pull request #17 from vincentbernat/fix/cleanexit

zkfarmer: end ZK connection cleanly at exit