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/disable_at.yaml b/tests/fixtures/layouts/disable_at.yaml
index 2956ebf..8e352d8 100644
--- a/tests/fixtures/layouts/disable_at.yaml
+++ b/tests/fixtures/layouts/disable_at.yaml
@@ -19,7 +19,7 @@
     name: project-test1
     nodes:
       - name: controller
-        image: image1
+        label: label1
 
 - project:
     name: org/project