Ensure zookeeper node exist to avoid throw Exception at startup

If the $zk_path/state node isn't exist, mostly at first time startup or has no
job submitted, chronos will throw an exception at start up when loading
jobs/tasks.

Signed-off-by: Chengwei Yang <yangchengwei@qiyi.com>
Remove unused zooKeeperLeaderPath

We in fact use zooKeeperCandidatePath as leader path, so remove this one which
never be used and will be used.

Signed-off-by: Chengwei Yang <yangchengwei@qiyi.com>