[frontend] allow us to create temporary aliases for chroots
See #727 (comment-568671)
See BZ 1705912
This change will allow us to simply solve the BZ:1705912 by configuring
`CHROOT_NAME_RELEASE_ALIAS` to `{"epel-8": "rhelbeta-8"}`, which will
effectively redirect requests for "epel-8" repo to "rhelbeta-8" repo.