(SERVER-113) Refactor whitelist logic out to ringutils

We are going to use the same whitelist logic for the new admin
endpoint that we were using for the cert-status endpoint.  This
commit refactors the logic out to the common ringutils namespace
so that we can access it from the new code.
(SERVER-113) Add new puppet-admin-api HTTP endpoint

This commit adds a new TK service and HTTP endpoint for puppet
admin functionality.  For now, it just has a placeholder
"/hello" route, but the scaffolding is in place for wiring
in the real functionality, including:

* Support for client whitelist configuration
* Versioned routes (just /v1 for now)