[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.
BuildConfig (and eventually anything that has hooks) should probably have one or more secret values that users can change, yes.  This will be the "standard" way we enable this.  For now, use a hardcoded "secret" value and we'll change it in a subsequent pull request.