callback hook for launchers

[minrk]

It would be great, when possible, to use a callback hook in launchers to signal when a launcher is ready. This could be used to return connection info from the controller to a Cluster object without needing access to the filesystem.

Unfortunately, we can't in general assume the controller or engines can call back to the host requesting the launch. So we need to keep the 'watch for files and copy them when they show up' behavior available, and perhaps as the default. However, for the simple Local launcher case, network callbacks should work nicer.