Make riffraff work with multibranch pipelines

Jenkins has support for so called multibranch pipelines. go-jenkins also supports this feature. Unfortunately I didn't get around to add that functionality to riffraff. As a result, multibranch pipelines show up with a yellow question mark when running "status", a build can't be triggered, and the logs cannot be printed.

As I use multibranch pipelines a lot at work, that would be a great feature to have.

What we need to do is use the go-jenkins features for multibranch pipelines and test it against a multibranch pipeline. I could do the testing on my side or we use any other Jenkins for it. Of course, tests are also an option.

If somebody wants to tackle this, that would be awesome. 😊