Implement a config-update CLI command

[chelma]

Description

As part of the work to implement the RFC for our revamped config handling (#80, #81), this task is the work to add a config-update command to update the capture/viewer node configuration, per the RFC. The steps should look something like:

* Create `config-update` command to update the existing config with new changes
* Implement/test rollback behavior
* README update

Acceptance Criteria

• config-update implemented; should synchronously update the configuration on the capture and viewer nodes
• config-update should not touch the capture/viewer nodes unless there is new/different config or behavior
• config-update should gracefully roll back if there is a bad configuration
• README updated to explain how to update configuration using the new command

[chelma]

Acceptance criteria met; resolving.