Switch to fedora-latest for testing

We'd like to remove fedora-27 from openstack-infra, using
fedora-latest will make it easier in the future to upgrade fedora
jobs.

Change-Id: I26d659f8c633e8757544741b6f6a4d76b0a76902
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

zuul.d/jobs.yaml

@@ -51,7 +51,7 @@
     vars:
       python_test_version: py35
       ansible_test_version: 2.6.0
-    nodeset: fedora-27
+    nodeset: fedora-latest
 
 - job:
     name: ara-integration-fedora-py35-devel
@@ -59,4 +59,4 @@
     vars:
       python_test_version: py35
       ansible_test_version: devel
-    nodeset: fedora-27
+    nodeset: fedora-latest