Directory: fix concurreny issue

If more than one thread would call _get_plugin_directory at the
same time and there was no plugin created then this may lead to
a race and each thread may have a different list of plugin data.

Change-Id: Id5d23f06e5f59fc07b660ef6c23cacf8cde1b999

neutron_lib/plugins/directory.py

@@ -66,7 +66,8 @@ _PLUGIN_DIRECTORY = None
 @_synchronized("plugin-directory")
 def _create_plugin_directory():
     global _PLUGIN_DIRECTORY
-    _PLUGIN_DIRECTORY = _PluginDirectory()
+    if _PLUGIN_DIRECTORY is None:
+        _PLUGIN_DIRECTORY = _PluginDirectory()
     return _PLUGIN_DIRECTORY