Automate contributor check during release pipeline.

This is a follow-up to Jiri's recommendation in DBZ-1259:

1) Commit the script
2) Have the script to return the exit code depending if all contirbutors are in or not
3) We would then incroporate the script into pre-release checks of release pipeline - https://github.com/debezium/debezium/blob/master/jenkins-jobs/scripts/release-pipeline.groovy