add plugins and config for ghprb

This requires the following new configuration settings:

* `jenkins::github::username`
  * In our case: `ros-pull-request-builder`
* `jenkins::github::api_url`
  * In our case: `https://api.github.com` (or left empty since that is the default)
* `jenkins::github::token`
  * In our case: `xxx` (please fetch token from existing buildfarm configuration