Package core plugins in Gerrit war and install them on init

This change adds a new Maven project 'gerrit-package-plugins' which
adds the core plugin jars to the Gerrit war file. Inside of the
resulting Gerrit war file the plugins are stored under
'WEB-INF/plugins/'.

The init command will now look at this folder during the site
initialization and offer the plugins for installation.

Change-Id: Ia6c28ef13bbbb7a0358c84e785b8422a2e6a47b3

gerrit-server/src/main/java/com/google/gerrit/server/plugins/PluginLoader.java

@@ -148,6 +148,11 @@ public class PluginLoader implements LifecycleListener {
     }
   }
 
+  public static File storeInTemp(String pluginName, InputStream in,
+      SitePaths sitePaths) throws IOException {
+    return asTemp(in, tempNameFor(pluginName), ".jar", sitePaths.tmp_dir);
+  }
+
   private static File asTemp(InputStream in,
       String prefix, String suffix,
       File dir) throws IOException {