ipaclient: add client-side command override class

This adds a new ipaclient.frontend module with two classes, CommandOverride
and MethodOverride, which can be used to implement additional client-side
functionality on top of server-side Command and Method plugins.

https://fedorahosted.org/freeipa/ticket/4739

Reviewed-By: David Kupka <dkupka@redhat.com>

ipaclient/frontend.py

@@ -0,0 +1,63 @@
+#
+# Copyright (C) 2016  FreeIPA Contributors see COPYING for license
+#
+
+from ipalib.frontend import Command, Method
+
+
+class CommandOverride(Command):
+    def __init__(self, api):
+        super(CommandOverride, self).__init__(api)
+
+        next_class = api.get_plugin_next(type(self))
+        self.next = next_class(api)
+
+    @property
+    def doc(self):
+        return self.next.doc
+
+    @property
+    def NO_CLI(self):
+        return self.next.NO_CLI
+
+    @property
+    def topic(self):
+        return self.next.topic
+
+    def _on_finalize(self):
+        self.next.finalize()
+
+        super(CommandOverride, self)._on_finalize()
+
+    def get_args(self):
+        for arg in self.next.args():
+            yield arg
+        for arg in super(CommandOverride, self).get_args():
+            yield arg
+
+    def get_options(self):
+        for option in self.next.options():
+            yield option
+        for option in super(CommandOverride, self).get_options():
+            if option.name not in ('all', 'raw', 'version'):
+                yield option
+
+    def get_output_params(self):
+        return self.next.output_params()
+
+    def _iter_output(self):
+        return self.next.output()
+
+
+class MethodOverride(CommandOverride, Method):
+    @property
+    def obj_name(self):
+        return self.next.obj_name
+
+    @property
+    def attr_name(self):
+        return self.next.attr_name
+
+    @property
+    def obj(self):
+        return self.next.obj