ZooKeeper SyncManager

Today Ironic does database-backed locking for various things. Currently, there is a column in the node table, which a process can put its hostname in to "lock" the node so that other processes will not attempt to mess with the data. While this works, it is vulnerable to deadlock scenarios when a conductor is partitioned or dies, which can lead to nodes being locked in the database until a third party (be it human or script) intervenes and removes the lock. Apache ZooKeeper has built in mechanisms that allow for implementation of a safer locking mechanism that releases locks upon client disconnect. The purpose of this spec is to implement ZooKeeper backed locking.