White 13 09 acu134762 add ghprb plugin

This is the initial implementation of the Github Pull Request Builder Plugin. Here are the changes covered in this pull request

1. Re-arranged attributes so there is a second tier heading called 'config' under 'jenkins' where all the configuration attributes will reside. This affects Vagrantfile, .kitchen.yml and the hosted chef role.
2. Add 'ghprb' to Jenkins plugin list.
3. Add new template to configure jenkins url and admin email; attributes to go with it.
4. Add new template to configure ghprb; attributes to go with it.
5. Modifications of the job template to support ghprb, plus changes to support it such as when the build is triggered, post build steps, utilizing web hooks, etc.