conf: fail if the deployment type is invalid

Created by: ggilmore

This PR shuts down the server if the supplied deployment type isn't one that we recognize.

Fixes: https://github.com/sourcegraph/sourcegraph/issues/458

Don't merge until https://github.com/sourcegraph/deploy-sourcegraph/pull/114 and https://github.com/sourcegraph/deploy-sourcegraph/pull/115 are approved (and their respective releases have been cut), and https://github.com/sourcegraph/infrastructure/pull/676 has been approved.