Provide a data directory to plugins on demand

If a plugin requests a data directory with a @PluginData File
dependency, a data directory will be created automatically by the
server in $site_path/data/$plugin_name and passed to the plugin.

Plugins can use this to store any data they want.

Change-Id: I98de44a033889740cafc8a3d72c582c0e99b7ffd

gerrit-pgm/src/main/java/com/google/gerrit/pgm/init/SitePathInitializer.java

@@ -72,6 +72,7 @@ public class SitePathInitializer {
     mkdir(site.mail_dir);
     mkdir(site.static_dir);
     mkdir(site.plugins_dir);
+    mkdir(site.data_dir);
 
     for (InitStep step : steps) {
       step.run();