Change node 'image' to 'label'

We are keeping the 'label' terminology in nodepool, and since that
is actually what zuul asks for, use that terminology here to avoid
confusion.

Change-Id: I5f5f1cd041b6dbd80a75af66fe02520f3eb32151
diff --git a/tests/fixtures/layouts/timer.yaml b/tests/fixtures/layouts/timer.yaml
index 95199e7..883c32e 100644
--- a/tests/fixtures/layouts/timer.yaml
+++ b/tests/fixtures/layouts/timer.yaml
@@ -28,13 +28,13 @@
     name: project-bitrot-stable-old
     nodes:
       - name: static
-        image: ubuntu-xenial
+        label: ubuntu-xenial
 
 - job:
     name: project-bitrot-stable-older
     nodes:
       - name: static
-        image: ubuntu-trusty
+        label: ubuntu-trusty
 
 - project:
     name: org/project