ipalib.messages: Add "message" property to PublicMessage

In Python 3, the "message" property was removed in favor of calling str().
Add it to PublicMessage, since IPA code depends on it.

Reviewed-By: Tomas Babej <tbabej@redhat.com>

ipalib/messages.py

@@ -122,6 +122,11 @@ class PublicMessage(UserWarning):
             code=self.errno,
         )
 
+    if six.PY3:
+        @property
+        def message(self):
+            return str(self)
+
 
 class VersionMissing(PublicMessage):
     """