debian: Change REPO_CHROOT_DIR to 'chroot-debian' from chroot-jessie

It was confusing to see a chroot-debian directory appear for a DIST that
was not being built so changed the directory name to something more
generic.

Also soft links the directory to whichever DIST is currently being built
so multiple DISTs can still share the same REPO_CHROOT_DIR
nrgaway:  Refactored 'custom' configuration files to be able to handle more generic situations
Added qubes-rpms-mirror-repo/repodata/ to .gitignore