Add a test for `dcos cluster list`
Add a test for `dcos cluster list`

It lists multiple clusters with different states and makes sure they are
displayed in the right order.

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