plugable: Move plugin base class and override logic to API

Each API object now maintains its own view of registered plugins. This change
removes the need to register plugin base classes.

This reverts commit 2db741e847.

https://fedorahosted.org/freeipa/ticket/3090
https://fedorahosted.org/freeipa/ticket/5073

Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>

ipaserver/advise/base.py

@@ -19,14 +19,12 @@
 
 import os
 from ipalib import api
-from ipalib.plugable import Plugin, Registry, API
+from ipalib.plugable import Plugin, API
 from ipalib.errors import ValidationError
 from ipapython import admintool
 from textwrap import wrap
 from ipapython.ipa_log_manager import log_mgr
 
-register = Registry()
-
 
 """
 To add configuration instructions for a new use case, define a new class that
@@ -97,7 +95,6 @@ class _AdviceOutput(object):
         self.content.append(line)
 
 
-@register.base()
 class Advice(Plugin):
     """
     Base class for advices, plugins for ipa-advise.