Refactor gitlab-runner-helper command registration
We currently use common.RegisterCommand to register runner commands, such as exec, list, run
etc. We use common.RegisterCommand2 to register gitlab-runner-helper
commands, such as artifacts-downloader, health-check
etc. At the same time we use common.GetCommands to register the commands in the Runner and in the helper which is very confusing and took me longer than I would like to admit to figure out.