Update test README to reflect changes in DC/OS cli. (#5923)
Update test README to reflect changes in DC/OS cli. (#5923)

Summary:
The new CLI has the dcos cluster setup command which is doing the
authentication. This patch reflects these CLI changes in the docs.
janisz/marathon,meln1k/marathon,mesosphere/marathon,guenter/marathon,meln1k/marathon,janisz/marathon,mesosphere/marathon,mesosphere/marathon,janisz/marathon,gsantovena/marathon,janisz/marathon,guenter/marathon,mesosphere/marathon,meln1k/marathon,janisz/marathon,gsantovena/marathon,gsantovena/marathon,guenter/marathon,mesosphere/marathon,gsantovena/marathon,guenter/marathon,guenter/marathon,meln1k/marathon,meln1k/marathon,gsantovena/marathon