[wip] Fail fast without an explicit monitoring setup.

Require some kind of explicit monitoring setup for `site.yml` - either enable PagerDuty, enable email, or disable monitoring entirely. At the `site.yml` level, we can specify which with `--extra-args`.

An implementation of https://github.com/rackerlabs/developer.rackspace.com/issues/518#issuecomment-52343622.