Don't fail on trying to delete non-existant images

This currently throws an attribute exception, which is definitely not
the right choice.

Change-Id: I2e08ce9f46f4fa8ec1d42508efb16a014d485850

releasenotes/notes/false-not-attribute-error-49484d0fdc61f75d.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - delete_image used to fail with an AttributeError if an invalid image
+    name or id was passed, rather than returning False which was the
+    intent. This is worthy of note because it's a behavior change, but the
+    previous behavior was a bug.

shade/openstackcloud.py

@@ -2820,6 +2820,8 @@ class OpenStackCloud(_normalize.Normalizer):
             self, name_or_id, wait=False, timeout=3600,
             delete_objects=True):
         image = self.get_image(name_or_id)
+        if not image:
+            return False
         with _utils.shade_exceptions("Error in deleting image"):
             # Note that in v1, the param name is image, but in v2,
             # it's image_id
@@ -2843,7 +2845,8 @@ class OpenStackCloud(_normalize.Normalizer):
                     "Timeout waiting for the image to be deleted."):
                 self._get_cache(None).invalidate()
                 if self.get_image(image.id) is None:
-                    return
+                    break
+        return True
 
     def _get_name_and_filename(self, name):
         # See if name points to an existing file