Enable cluster-wide CORS Expose-Headers setting

An operator proposing a web UX to its customers might want to allow web
browser to access some headers by default (eg: X-Storage-Policy,
 X-Container-Read, ...). This commit adds a new setting to the
proxy-server to allow some headers to be added cluster-wide to the CORS
header Access-Control-Expose-Headers.

Change-Id: I5ca90a052f27c98a514a96ee2299bfa1b6d46334

etc/proxy-server.conf-sample

@@ -73,6 +73,9 @@ bind_port = 8080
 # cors_allow_origin =
 # strict_cors_mode = True
 #
+# Comma separated list of headers to expose through Access-Control-Expose-Headers
+# cors_expose_headers =
+#
 # client_timeout = 60
 # eventlet_debug = false
 #