Proxy requests to candlepin for guest ids and content overrides API calls

Open up routes to candlepin to support content overrides and new guestid urls.

Subscription Manager needs the content overrides urls accessible to enable its repo management capabilities.

Candlepin now exposes a new API for updating a host's guest ids (outside of update consumer).

Two 'fake'  api paths were also added as subscription manager checks their existence in order to determine if the functionality is enabled.