Extend help for openQA client config, e.g. mention the environment variable OPENQA_CONFIG
Many users use API key and secret command line parameters because they are not aware of the config file or the according format. Also bots are all using the same user, e.g. "geekotest", likely because people are not aware how OPENQA_CONFIG works, see https://github.com/os-autoinst/openQA/blob/master/lib/OpenQA/UserAgent.pm#L37 . We can improve the help.
--helpof openqa-client and openqa-cli make the information about the config file and OPENQA_CONFIG easily accessible
- Extend the help
- Consolidate the different comments and help strings we have
- Ensure the env variable OPENQA_CONFIG is mentioned in help+doc