Return a default plugin.

def get_default_plugin(cls):
        '''
        Return a default plugin.
        '''
        from importlib import import_module
        from django.conf import settings
        default_plugin = getattr(settings, 'ACCESS_DEFAULT_PLUGIN', "access.plugins.DjangoAccessPlugin")
        if default_plugin not in cls.default_plugins:
            logger.info("Creating a default plugin: %s", default_plugin)
            path = default_plugin.split('.')
            plugin_path = '.'.join(path[:-1])
            plugin_name = path[-1]
            DefaultPlugin = getattr(import_module(plugin_path), plugin_name)
            cls.default_plugins[default_plugin] = DefaultPlugin()
        return cls.default_plugins[default_plugin]