Fedora support is never tested, and has been unmaintained for a while.
Because we don't expect any actual user using OpenStack on Fedora, this
change drops support for Fedora directly.
This change also fixes the release version used in CentOS8/RHEL8 when
setting up package repositories.
Change-Id: I541a57bf4bdb51d1d27ff25e317683e3dbb56afa