config: only include oslo.middleware options that are shipped

Adding options from middleware not enabled by default is not a good
idea.

Change-Id: I3ed922c84e6d37f2424262d108e6bdc5a03e2858

etc/gnocchi/gnocchi-config-generator.conf

@@ -4,7 +4,8 @@ wrap_width = 79
 namespace = gnocchi
 namespace = oslo.db
 namespace = oslo.log
-namespace = oslo.middleware
+namespace = oslo.middleware.cors
+namespace = oslo.middleware.http_proxy_to_wsgi
 namespace = oslo.policy
 namespace = cotyledon
 namespace = keystonemiddleware.auth_token