The repo format has slightly changed for CentOS 8 (s/os/baseos/).
Make the chroot builder look for a more specific repos.d directory
first named for the distro variable, then fall back to to top-level
dir (this avoids having to constantly change fedora).
Update the gate mirror setup and roles for new Centos 8 paths too.
Change-Id: I5b7f0c3624cac1d7aa7ed8bf6286b85d808b9c9a
