search play.plugins descriptor in application path

* it's important when <application dir> is not the same as <working dir>
* in other words, when admin starts the application with a command like `cd /tmp && play start ~/hello`

Author: asolntsev

framework/src/play/plugins/PluginCollection.java

@@ -196,7 +196,7 @@ List<URL> loadPlayPluginDescriptors() {
         try {
             String playPluginsDescriptor = Play.configuration.getProperty("play.plugins.descriptor");
             if (playPluginsDescriptor != null) {
-                return Collections.singletonList(new File(playPluginsDescriptor).toURI().toURL());
+                return Collections.singletonList(new File(Play.applicationPath, playPluginsDescriptor).toURI().toURL());
             }
             return Collections.list(Play.classloader.getResources(play_plugins_resourceName));
         }