Ignores image pull secret if not passed

[scardena]

This gets rid of completetly setting the option of imagePullSecret when we don't pass any image credentials. right now it's being set to null, and it's setting the imagePullSecret: null but it could be that journalctl also picks that up and logs it as an error.

• adds a simple test to make sure that no imagePullSecret should be set if no creds are passed
• adds the check for the post-install job as well.
• adds readme to run tests locally