Help text for command line arguments

[silvanmelchior]

So far, there is no way of showing all available command line options if ConfZCLArgSource is used. It would be great to have an option to show a helper text to the user, similar to what is known from Python's argparse.

The same could be realised for ConfZEnvSource by showing a list of supported variables.

This feature could also be integrated with validate_all_config, to show a helper message if the configuration is not valid.