Make the current cluster configurable for tests
Make the current cluster configurable for tests

This is done by adding a new field in the mock context.

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