Keystone is now using the HTTPProxyToWSGI middleware from
oslo.middlware in its default api-paste configuration [1]. This commit
gives us the ability to enable/disable that middlware.
[1] Iad628a863e55cbf20c89ef23ebc7527ba8e1a835
Change-Id: I0fec98a6e1d9c8be4d8b8df382b78ba2815790f9