Switch to fedora-latest for nodeset name

To help avoid the amount zuul.yaml chrun when we bring a new version
of fedora online, switch to using fedora-latest. As of writing,
fedora-28 is the latest release which we update our testing for.

Also add fedora-28 support to stash.sh and remove fedora-25 /
fedora-26 as they are EOL.

Change-Id: I3d716554e8f270f4434cc9cac3408f8e890e0665
Depends-On: https://review.openstack.org/565758/
Signed-off-by: Paul Belanger <pabelanger@redhat.com>
2 files changed