Fixes #5968: Intercept candlepin request to get available releases

Intercept requests to GET /consumers/:id/available_releases and return
the system's available release versions as viewed by katello.

Added /api/available_releases to the root resource so that RHSM
can check to see if it is enabled on the server.

This API is only intended to be called by a registered consumer.
Merge pull request #4181 from mstead/Fixes-5968-release-versions

Fixes #5968: Intercept candlepin request to get available releases