Tolerate different proxy pass configurations

The stock reverse proxy configuration strips the leading
'/confluent-api/' from the URL.

However, when doing a custom reverse proxy set up, one may preserve full
path without knowing which way to go.

Since '/confluent-api/' will never be used inside the api, just strip
it when detected to tolerate either of the likely proxy pass behaviors.

confluent_server/confluent/httpapi.py

@@ -628,6 +628,8 @@ def wsock_handler(ws):
 
 
 def resourcehandler(env, start_response):
+    if env['PATH_INFO'].startswith('/confluent-api'):
+        env['PATH_INFO'] = env['PATH_INFO'].replace('/confluent-api', '')
     for hdr in env['headers_raw']:
         if hdr[0].lower().startswith('confluent'):
             hdrname = hdr[0].upper()