Gate latest Fedora by using the fedora-latest nodeset

Fedora release new releases pretty often and the infrastructure
team does not want to maintain multiple images of Fedora.  Therefore,
they are suggesting that we use the `fedora-latest` nodeset to always
get the latest Fedora release. This is similar to the `debian-stable`
nodeset.

Change-Id: Ib19b9c5ff061e5250d65e4e397e599d70995fc76

zuul.d/jobs.yaml

@@ -19,6 +19,6 @@
     nodeset: debian-stable
 
 - job:
-    name: openstack-ansible-functional-fedora-27
+    name: openstack-ansible-functional-fedora-latest
     parent: openstack-ansible-functional
-    nodeset: fedora-27
+    nodeset: fedora-latest

zuul.d/project.yaml

@@ -18,8 +18,8 @@
     check:
       jobs:
         - openstack-ansible-functional-debian-stable
-        - openstack-ansible-functional-fedora-27
+        - openstack-ansible-functional-fedora-latest
     gate:
       jobs:
         - openstack-ansible-functional-debian-stable
-        - openstack-ansible-functional-fedora-27
+        - openstack-ansible-functional-fedora-latest