Pecan: add http_proxy_to_wsgi middleware

Pecan does not currently use api paste to load middleware like the
legacy wsgi, so we need to explicitly import it and wrap the pecan app.
This is a follow-up to Ice9ee8f4e04050271d59858f92034c230325718b.

Change-Id: I3e1b08bf1f902cf09c8a39699c00f1b0d22c3277
Closes-Bug: #1590608

neutron/pecan_wsgi/app.py

@@ -17,6 +17,7 @@ from keystonemiddleware import auth_token
 from neutron_lib import exceptions as n_exc
 from oslo_config import cfg
 from oslo_middleware import cors
+from oslo_middleware import http_proxy_to_wsgi
 from oslo_middleware import request_id
 import pecan
 
@@ -80,6 +81,9 @@ def _wrap_app(app):
     # version can be unauthenticated so it goes outside of auth
     app = versions.Versions(app)
 
+    # handle cases where neutron-server is behind a proxy
+    app = http_proxy_to_wsgi.HTTPProxyToWSGI(app)
+
     # This should be the last middleware in the list (which results in
     # it being the first in the middleware chain). This is to ensure
     # that any errors thrown by other middleware, such as an auth