XD-1591: Flatten out ephemeral nodes

Containers that write out ephemeral nodes indicating deployment of a module previously used the following node structure for streams:

`/xd/deployments/streams/<name>/<type>/<label>/<container>`

This has been modified to look like this:

`/xd/deployments/streams/<name>/<type>.<label>.<container>`

This change will make it easier to create ZooKeeper watches on the stream to detect the addition/removal of modules and thus calculate the state of the stream.

It is recommended that existing streams and jobs in ZooKeeper are **destroyed** before using this change. Otherwise it is recommended to clear out the `/xd` path in ZooKeeper before usage.
Could use Assert.state() here (since IllegalStateException is desired) or Assert.hasText(..) (which would instead throw IllegalArgumentException). In either case though, the 6 lines could be replaced by 2, and it improves test coverage (unless we have tests that make it into each of these if blocks.