CLI expansion: stateful behaviour and more command feedback

Problem Statement

The CLI currently requires each parameter to be set in each command, including the controller. The only exceptions are predefined defaults for the controller and gNMI target. This results in very lengthy commands for local testing. Furthermore, the "init" command gives no output for success.

Proposed solution

Expand the CLI to save and reuse parameters to and from the config at runtime. For example, save the given controller and reuse until changed by user. Analyze current CLI outputs and improve where possible.