(SERVER-113) admin 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)



IIRC, you were discussing consolidating this config section with the `/certificate_status` whitelist ... something along the lines of:
```json:
access-control: {
    # whitelists go in here
}
```
Did you decide against that?