plugable: initialize plugins on demand

Use a new API namespace class which does not initialize plugins until they
are accessed.

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

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

ipatests/test_ipalib/test_cli.py

@@ -87,7 +87,7 @@ class DummyCommand(object):
 
 class DummyAPI(object):
     def __init__(self, cnt):
-        self.__cmd = plugable.NameSpace(self.__cmd_iter(cnt))
+        self.__cmd = plugable.APINameSpace(self.__cmd_iter(cnt), DummyCommand)
 
     def __get_cmd(self):
         return self.__cmd