Use fedora-29 instead of fedora-28

We are very close to being able to remove our fedora-28 images as this
release is now EOL. Use fedora-29 in the openshift job so that we can
remove fedora-28 without affecting nodepool's testing.

Change-Id: I1983d9408d038a9431159d75f1d6e2dc47a0456f

.zuul.yaml

@@ -120,7 +120,7 @@
         - name: cluster
           label: centos-7
         - name: launcher
-          label: fedora-28
+          label: fedora-29
     required-projects:
       - zuul/nodepool