Github - Require webhook_token

It's quite unsafe to run without webhook_token, and quite easy for us to
run our tests with a terribly predictable one. This will ensure that
nobody accidentally runs a Zuul vulnerable to MITM proxy attacks.

Per the link right under the doc we just changed, we also use
hmac.compare_digest to prevent timing analysis by malicious attackers
which would help them discover the secret.

Change-Id: Ie8aa83b81b8e4ef1bb755a664bf416a8663930fa
6 files changed