registry: fix non-unique default dataSources

Fix the problem where `registry.defaultDataSources` has two instances:

 - `require('loopback').defaultDataSources` used by
   `loopback.autoAttach()`

 - `require('./registry').defaultDataSources` used by
   `app.dataSource`.

I am intentionally leaving out unit-tests as the whole `autoAttach`
feature is going to be deleted before 2.0 is released.

lib/registry.js

@@ -13,6 +13,7 @@ var DataSource = require('loopback-datasource-juggler').DataSource;
 
 var registry = module.exports;
 
+registry.defaultDataSources = {};
 
 /**
  * Create a named vanilla JavaScript class constructor with an attached
@@ -264,8 +265,7 @@ registry.memory = function (name) {
  */
 
 registry.setDefaultDataSourceForType = function(type, dataSource) {
-  var defaultDataSources = this.defaultDataSources ||
-    (this.defaultDataSources = {});
+  var defaultDataSources = this.defaultDataSources;
 
   if(!(dataSource instanceof DataSource)) {
     dataSource = this.createDataSource(dataSource);