support equivalent of [merge] comment in pipelines

[rawlingsj]

fabric8's own CI jobs use the github pull request plugin and a [merge] on PRs to merge changes providing tests pass. This doesn't work with Jenkins Pipelines so it means we are still reliant on the old CI infra.

I wonder if instead of the flow.addMergeCommentToPullRequest(id, project) steps we have in the library we could have a new function that checks the PR has had CI checks passed and then performs a merge?

https://developer.github.com/v3/pulls/#merge-a-pull-request-merge-button