Add capability to parse forward headers

In case standalone watcher-api runs behind forwarders (like load
balancers), it should parse specific request headers to determine
the endpoint url clients actually use.

Add http_proxy_to_wsgi middleware to api pipeline to handle this.

Closes-Bug: #2122353
Change-Id: I27ade17f7ce1649295f92f3ea1af620df63ba1bc
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

etc/watcher/oslo-config-generator/watcher.conf

@@ -9,6 +9,7 @@ namespace = oslo.concurrency
 namespace = oslo.db
 namespace = oslo.log
 namespace = oslo.messaging
+namespace = oslo.middleware.http_proxy_to_wsgi
 namespace = oslo.policy
 namespace = oslo.reports
 namespace = oslo.service.periodic_task