plugable: remember overriden plugins in API
    
    Remember what plugin class was overriden by what plugin class in API
    objects. Add new method API.get_plugin_next which returns the plugin
    class which was overriden by the plugin class specified as argument.
    
    https://fedorahosted.org/freeipa/ticket/4739
    
    Reviewed-By: David Kupka <dkupka@redhat.com>