tests: Discover absence of zookeeper faster

It turns out that when no zookeeper server is available for tests
python ./setup.py test spends almost half of it's running time
looking for it. This change addresses that and makes unit tests
run roughly two times faster on my zookeeperless machine:

- client starting timeout reduced from default 15 seconds to 3 seconds,
  which should be more than enough to connect to localhost;
- zookeeper availability is checked only once, on module import,
  and then saved for later re-use.

Change-Id: I11932e7cca12436cde0bcf0a20cb2adb96fbb27a