[WIP] Allow git webhooks to trigger build on git push - Card#287

This is initial part of my work. I've created the main players of this story, meaning:
* updated openshift v3 docs, I've added `/build/schedule` node
* main part of the webhooks responsible for registering, managing and feeding plugins with data

@smarterclayton @bparees please review.
If we're not actually using any of this data we should only declare the fields we are using.  Also, these are tied to a version so name them "V3".  Would remove all the fields we're not using.