fixes #8188 - adding api to determine what inc update is needed

It think could but I won't hold up your PR on following a convention.

If this API lives at `/systems/bulk/available_incremental_updates` and only handles calculating errata, what will the URL for packages be?  Will it live here and accept a `:package_ids` param or should we make this `/systems/bulk/available_incremental_errata` or some such?