The CLI treats `airflow connection` as a single command, with `--list`, `--add`, etc. as flags. This means it's possible to pass options that can't be used together: passing `--list` with `--conn_id` should be invalid. The current implementation has to handle validation of mutually exclusive options separately for each command. I think the code would be simpler and easier to use if we used nested commands instead of flags: `airflow connections list` and `airflow connections add` would be separate subcommands that would take different arguments, and we wouldn't have to check for invalid combinations of commands and arguments.
This might overlap with other CLI refactoring, like https://issues.apache.org/jira/browse/AIRFLOW-3358. I'm not sure if that conversation is still active, though. Interested to get feedback about this--maybe there are advantages to using flags instead of subcommands that I haven't thought of.
