(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)



We already have one of these in the `certificate-authority` service here: https://github.com/puppetlabs/puppet-server/blob/master/src/clj/puppetlabs/services/ca/certificate_authority_core.clj#L115.  I understand not wanting to create a dependency between these two services but might be good to put this in a common location to avoid the repetition.  Maybe we should add this to the JVM-CA?