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/unit/test_v3.py b/tests/unit/test_v3.py
index d15189d..0763dd0 100644
--- a/tests/unit/test_v3.py
+++ b/tests/unit/test_v3.py
@@ -375,9 +375,9 @@
                 name: duplicate
                 nodes:
                   - name: compute
-                    image: foo
+                    label: foo
                   - name: compute
-                    image: foo
+                    label: foo
             """)
 
         file_dict = {'.zuul.yaml': in_repo_conf}
@@ -400,7 +400,7 @@
                 name: duplicate
                 nodes:
                   - name: compute
-                    image: foo
+                    label: foo
                 groups:
                   - name: group
                     nodes: compute