XD-1851 Introduce cache to ZooKeeperContainerRepository

Introduced PathChildrenCache to ZooKeeperContainerRepository
to address the concerns indicated in
https://github.com/spring-projects/spring-xd/pull/936

Container cache no longer needs to be a member

Since the container cache is no longer externally accessible
due to ZooKeeperContainerRepository, this was moved
to be a local variable within the leader election method.