Add ID() function to Cluster.
Add ID() function to Cluster.

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