This sets up the HTTPProxyToWSGI middleware in front of Mistral API. The
purpose of this middleware is to set up the request URL correctly in
the case there is a proxy (for instance, a loadbalancer such as HAProxy)
in front of the Mistral API.
The HTTPProxyToWSGI is off by default and needs to be enabled via a
configuration value.
It can be enabled with the option in mistral.conf:
[oslo_middleware]
enable_proxy_headers_parsing=True
Closes-Bug: #1590608
Closes-Bug: #1816364
Change-Id: I04ba85488b27cb05c3b81ad8c973c3cc3fe56d36
ca1acb656c