Initialize activation status

activation_status throws an exception if the image set fails
because it is not initialized.

Change-Id: Iff6d5a8844eed954fb1c0bcea96118b99ac6bcf4

openstackclient/image/v2/image.py

@@ -775,6 +775,7 @@ class SetImage(command.Command):
         image = utils.find_resource(
             image_client.images, parsed_args.image)
 
+        activation_status = None
         if parsed_args.deactivate:
             image_client.images.deactivate(image.id)
             activation_status = "deactivated"