Expose setting for --retain-old-count in repos
By default katello retains all old versions of packages in a repo. This is fine for now but will eventually fill up drive space.
Even if products stop updating (6Server, 6epel, etc.) we will want to start syncing new products (8Server) which will still fill up our disk space.
There needs to be an easy way to trim old content so drive space doesn't fill up indefinitely.