Multibranch whitelist

Adding ability for GHPRB to specify a whitelist of target (destination) branches that should be tested. If you specify branches for this whitelist, then only pull requests destined for those branches will be tested.

Worked with David Wang on this, many thanks to his insight into how to use repeatable fields.

This is to implement https://issues.jenkins-ci.org/browse/JENKINS-18737 .