Supports multiple ipclusters via the --cluster-id option
  - flexibly way to choose how ipclusters restart/stop/start via ClusterID config
    - this provides users with option to not kill active clusters
  - backwards compatible

TODO:
  - update starcluster documentation
  - option to specify ClusterID class in starcluster config
  - ln -s ~/.ipython/profile_default/security/{<current json>,ipcontroller-[engine|client].json}
can now specify ClusterID class in starcluster config
symlink current cluster_id to ipcontroller-client.json for backwards compatibility.  nothing exists to remove this if the current cluster dies, though that doesn't negatively affect new clusters created.

lint - remove 80+ column lines

change json symlink to cp