Now that service-name is optional I believe that we should display a list of the available roles (with a small description) instead of service names.
$ ./bin/whirr Usage: whirr COMMAND [ARGS] where COMMAND may be one of: version Print the version number and exit. launch-cluster Launch a new cluster running a service. destroy-cluster Terminate and cleanup resources for a running cluster. list-cluster List the nodes in a cluster. Available roles for instances: nn hadoop name node jt hadoop job trackers ...