Create missing PR during synchronize trigger

When the GitHub syncronize trigger is sent (e.g. adding commits to a pull request) it can fail if Jenkins didn't previously know about the PR.  This can happen, for example, if Jenkins missed an `opened` notification, or if the PR was created before GHPRB was enabled.

The result is a message in the Jenkins log like:

    Pull Request #XX doesnt exist

This PR fixes the issue by creating the pull request if it is missing.