Replace assert with condition

a piece of code in image client has some business logic behind
assert, which can be lost when running python in optimized mode (-O).

Change-Id: I2179970df495e1215d691915c51cebe5cb4541a7

openstackclient/image/v2/image.py

@@ -1024,9 +1024,7 @@ class UnsetImage(command.Command):
 
         if parsed_args.properties:
             for k in parsed_args.properties:
-                try:
-                    assert(k in image.keys())
-                except AssertionError:
+                if k not in image:
                     LOG.error(_("property unset failed, '%s' is a "
                                 "nonexistent property "), k)
                     propret += 1