Mention internet proxy format

tower/ui/datacenter_ui.js

@@ -107,7 +107,8 @@ var datacenter_form = {
                     view: "text",
                     name: "proxy",
                     label: "Internet Proxy",
-                    placeholder: "Proxy address:port"
+                    placeholder: "Proxy address:port",
+                    bottomLabel: "Format http://user:password@192.168.1.1:3128"
                 },
                 {}
             ]