frontent: Add summary class property to CommandOverride

Avoid creating instance of overriden command to get its summary.

https://fedorahosted.org/freeipa/ticket/6048

Reviewed-By: Jan Cholasta <jcholast@redhat.com>

ipaclient/frontend.py

@@ -126,6 +126,12 @@ class CommandOverride(Command):
 
     doc = classproperty(__doc_getter)
 
+    @classmethod
+    def __summary_getter(cls):
+        return cls.__get_next().summary
+
+    summary = classproperty(__summary_getter)
+
     @classmethod
     def __NO_CLI_getter(cls):
         return cls.__get_next().NO_CLI