Add http_proxy_to_wsgi to config-generator

The http_proxy_to_wsgi was recently added, however, in order to have
this configuration added by the config generator, we need to add it
to that relevant conf file.

Change-Id: Ie763367a75cde2deff8fcdfda47e08ce990bae00
(cherry picked from commit 9191874c3f)

etc/ceilometer/ceilometer-config-generator.conf

@@ -7,6 +7,7 @@ namespace = oslo.db
 namespace = oslo.log
 namespace = oslo.messaging
 namespace = oslo.middleware.cors
+namespace = oslo.middleware.http_proxy_to_wsgi
 namespace = oslo.policy
 namespace = oslo.service.service
 namespace = keystonemiddleware.auth_token