sheep/zookeeper: ensure zookeeper is connected

zookeeper session establishment is asynchronous. It means that
we cannot judge the session is connected by checking the
return value of the func zookeeper_init. Instead, we should
check the zookeeper handler's state in a loop to ensure
the session is connected.

Signed-off-by: Ruoyu <liangry@ucweb.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>