repos: simplify how repos are set up.

removes lots of the complexity used exclusively by GroupsCommand (which in
current DNF is defunct anyway).
repos: get rid of Base.prerepoconf.

more repo setup simplification.
refactor: introduce Cli.command property.

Also rename do_commands() to run(), it makes more sense (also it is always
a single command).
cosmetic: refactor: getReposFromConf*() -> read_*_repos()

so it looks more like the Base.read_conf_file().