Add Integration Tests

[guettli]

/kind feature

Describe the solution you'd like

As a user of csmctl, I want to be sure that the command-line tool works as expected.

New features should not accidentally break existing features.

I would like to have a test which checks:

• the calculated hash values don't change with a new version of csmctl. The hash should only change if the content of the cluster-stack has changed.
• The modes (hash-mode, stable-mode, ...) work like expected
• The templating works correctly.

Anything else you would like to add:

Since csmctl is a command-line tool, we could write this test as a bash script. If we do so, we should use set -euxo pipefail to be sure that no errors are overlooked.