Keystone doesn't use http_proxy_to_wsgi middleware

[s10]

Keystone since migration to flask doesn't import nor use http_proxy_to_wsgi middleware.

How to reproduce:
1. Start Keystone with uwsgi as http application
2. Set [oslo_middleware] option enable_proxy_headers_parsing=true in keystone.conf
3. Setup SSL terminating reverse proxy, add headers X-Forwarded-Proto https
4. curl Keystone API version endpoint:
curl https://identity.example.com/

What is expected:
{
  "versions": {
    "values": [
      {
        "id": "v3.13",
        "status": "stable",
        "updated": "2019-07-19T00:00:00Z",
        "links": [
          {
            "rel": "self",
            "href": "https://identity.example.com/v3/"
          }
        ],
        "media-types": [
          {
            "base": "application/json",
            "type": "application/vnd.openstack.identity-v3+json"
          }
        ]
      }
    ]
  }
}

What is an actual result:
{
  "versions": {
    "values": [
      {
        "id": "v3.13",
        "status": "stable",
        "updated": "2019-07-19T00:00:00Z",
        "links": [
          {
            "rel": "self",
            "href": "http://identity.example.com/v3/"
          }
        ],
        "media-types": [
          {
            "base": "application/json",
            "type": "application/vnd.openstack.identity-v3+json"
          }
        ]
      }
    ]
  }
}

If we look at the code, Keystone flask application doesn't use oslo_middleware and application_url from the request, it only gets PATH_INFO from the environment, which can't be set in the reverse proxy:
https://github.com/openstack/keystone/blob/master/keystone/server/flask/common.py#L673