Improve handling of config.yaml

These patches make the handling of missing/invalid config.yaml files a bit better. The config is now validated as soon as the server is initialized, rather than validating individual config values only when they are used.

Unfortunately, Torquebox does not initialize the server until the first request is served; to get the desirable behavior of failing immediately on application start when the configuration is invalid, we may have to resort to running a job or some such thing ...