CLI Improvements for Configuration

[chelma]

Description

As part of the work to implement the RFC for our revamped config handling (#80, #81), this task implement a few quality-of-life improvements for working with configuration. The steps should look something like:

* Add new config-related commands and options:
    * `config-list`: get a listing of config bundles for a given cluster (e.g. what's in S3?)
    * `config-pull`: pull a specified config bundle from S3 to the client
    * `config-update --version`: a way to deploy a specific, historical version of the config to the cluster
* README update

[chelma]

This ticket has served its purpose and can now be closed. More detailed follow-ups and new features can be tracked here: #140