Update Aptly scripts

Modified the scripts create_repo.py, remove_repo.py, and update_repo.py to support multi-component package repositories. For example, it is now possible to manage a repository for the distribution bionic that contains both a testing component and an unstable component. The optional prefix has been made mandatory; it can be used to identify the team or project that manages the repository (e.g. schaap). Finally, the help message text is now much more descriptive.