Refactor Sveltosctl to use cobra as CLI framework

[struegamer]

Today sveltosctl is using github.com/docopt/docopt-go as CLI framework.

But docopt-go is a bit outdated, as you can see from it's github commit history.

Therefore I would propose a refactoring towards cobra CLI framework (https://cobra.dev/) as this lib is pretty much the standard CLI framework for GO CLI utils.

Furthermore it has a living community and we can move sveltosctl to be more compatible to certain kubectl patterns.

Some remarks:

1. It needs to be backwards compatible, means the output of all/certain commands are not changing
2. it needs to be easily testable, without relying on any k8s clusters, means mocking k8s api.
3. it needs to be easily extendable, eventually adding a plugin mechanism similar to kubectl.

This issue report should function as a general documentation about the progress.

[struegamer]

As I can not assign this issue to myself :) I am just stating here, that I am working on this :)

[gianlucam76]

Assigned to @struegamer

Thank you