As described in the comments, CentOS overrides the "distroverpkg"
variable in yum.conf. This is the package that yum queries to
establish the value of the $releasever variable. On other platforms,
this defaults to "redhat-release" (which "fedora-release" provides) so
everything works. It is only when the base-system "distroverpkg"
refers to a package not in the chroot we hit the issue.
We can avoid this by setting the releasever variable via the
commandline.
Change-Id: I231c3277960992cd479b8aff7838f246397936f2
ce781fbbc4