ciao-launcher: do not start docker plugin on NN

[mcastelino]

On NN nodes docker may not be provisioned. Hence it may not make sense to start the docker plugin or interact with Docker on the NN. (or start docker plugin only if docker is detected on the NN).

[markdryan]

Failure to start the docker plugin does not prevent launcher from starting up. All that will happen is that you will see a warning in the logs. Still no point it doing work that is not needed.