Adding tests for cluster_standalone.Cluster constructor.
Adding tests for cluster_standalone.Cluster constructor.

Previously untested the non-default versions of display_name
and serve_nodes (they were "covered" since they occurred on
single lines).

joerideturck/gcloud-python-bigtable,dhermes/gcloud-python-bigtable,dhermes/gcloud-python-bigtable,joerideturck/gcloud-python-bigtable