Fix inheritance of delete-after-upload option

This diskimage option was not correctly inherited when using
a parent diskimage.

Change-Id: If48c218a0b8b9432e54a55bd947b04016db78479

nodepool/config.py

@@ -348,7 +348,9 @@ class DiskImage(ConfigValue):
         keep_image_types = config.get('keep-formats', None)
         if keep_image_types:
             self.keep_image_types = set(keep_image_types)
-        self.delete_after_upload = config.get('delete-after-upload', False)
+        delete_after_upload = config.get('delete-after-upload', None)
+        if delete_after_upload is not None:
+            self.delete_after_upload = delete_after_upload
         pause = config.get('pause', None)
         if pause:
             self.pause = pause

nodepool/tests/fixtures/node_diskimage_cleanup_formats.yaml

@@ -61,10 +61,12 @@ providers:
             min-ram: 8192
 
 diskimages:
-  - name: fake-image
+  - name: parent
     delete-after-upload: true
     keep-formats:
       - qcow2
+  - name: fake-image
+    parent: parent
     elements:
       - fedora
       - vm