cf:help should not require complete configuration

[jtuchscherer]

Currently, running mvn cf:help produces an error message if I did not specify cf.target, cf.space and cf.org. That is slightly confusing, especially since the README states that - as a minimum - I can just pull in the plugin without any config.
Also, the artifact has to be built in order to run that command. Otherwise, it will through an error message. So, I could not just clone a git repo and run mvn cf:help. You have to run mvn package first.