Make concurrent requests when listing clusters
Make concurrent requests when listing clusters

When converting a Cluster into a dict, an HTTP request is being made in
order to get the cluster version.

This makes use of Python's `concurrent.futures` module to trigger
requests asynchronously.

kensipe/dcos-cli,kensipe/dcos-cli,dcos/dcos-cli,kensipe/dcos-cli,dcos/dcos-cli,dcos/dcos-cli,kensipe/dcos-cli,kensipe/dcos-cli,dcos/dcos-cli,dcos/dcos-cli