[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.
Actually, at least github allows secret to sign the payload being sent, this signature is then one of the POST headers: X-Hub-Signature - HMAC hex digest of the payload, using the hook’s secret as the key, if configured (see https://developer.github.com/webhooks/#payloads). But I'm not sure whether others have the same mechanism.
So I'd propose to change the secret to be randomly generated token. This is to make sure the webhook URLs are not easily guessable. This can be than regenerated on user demand, or for example each time when modifying webhook configuration. And the secret will still be a separate field optionally configured by the user. What do you think?