Bug: `--guarantee-final-pipeline` does not work with Gitlab CI

[qusielle]

--guarantee-final-pipeline option is the great thing to have within Marge-bot but it does not do more than commenting jenkins retry, unfortunately.

As it suggests from the title and the help, it should trigger a Gitlab CI pipeline and wait for it, but it does not happen.

I have proposed a pull request implementing the missing functionality, please check: #351