Jenkinsfile: alternative to using git commit author for building workflow-cli image

As seen in https://github.com/deis/controller-sdk-go/pull/115#issuecomment-277495248, the assumption that the git commit author is the same as the user who owns the forked repo can be proven faulty. (Logic where this is used in CI to build a workflow-cli image with changes in a controller-sdk-go PR can be seen [here](https://github.com/deis/controller-sdk-go/blob/master/Jenkinsfile#L127-L155))

We therefore seek an alternative way to fetch and inject a PR in this repo into a workflow-cli build artifact (via glide)

Read Jenkinsfile lines 127-155 and the linked pull request comment to understand how the commit author is used when building the workflow-cli image. Trace the glide-based dependency injection path and identify an alternative that fetches the pull request from this repository. Done means the CI build uses the pull request changes without assuming its commit author owns the fork.

Written by the indexing model from the issue text.