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/smtp.yaml b/tests/fixtures/layouts/smtp.yaml
index 8f53d02..fd91d36 100644
--- a/tests/fixtures/layouts/smtp.yaml
+++ b/tests/fixtures/layouts/smtp.yaml
@@ -46,14 +46,14 @@
     name: project-test1
     nodes:
       - name: controller
-        image: image1
+        label: label1
 
 - job:
     name: project-test1
     branches: stable
     nodes:
       - name: controller
-        image: image2
+        label: label2
 
 - job:
     name: project-test2