Fix #1564: Use Procfile for process types in Dockerfile apps

* Document new Procfile feature
* Add and update tests
* Fix a typo in generate_scale_file (ref #1923)

tests/apps/dockerfile-procfile/web.js

@@ -0,0 +1,12 @@
+var express = require('express');
+
+var app = express.createServer(express.logger());
+
+app.get('/', function(request, response) {
+  response.send('nodejs/express');
+});
+
+var port = process.env.PORT || 5000;
+app.listen(port, function() {
+  console.log("Listening on " + port);
+});