Allow target organisation override

[jonesbusy]

While doing interactive test on #295 I found that I cannot test opening PR on the jenkinsci organisation because we need an app installed by administrator. Until we are doing this I will test be creating some repo on my own org

This PR allow an environment var to be set to override the target organisation. Also add some checks and test to ensure the parent fork is the one expected

Testing done

Automated tests and checking logs

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue