Create plugin directory during init

Let's create the correct directory so admins don't have to worry about
typos.

Change-Id: Iec1cd77f568e3e94314d8ac647c9715acf972d40
Signed-off-by: Brad Larson <bklarson@gmail.com>

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

@@ -70,6 +70,7 @@ public class SitePathInitializer {
     mkdir(site.logs_dir);
     mkdir(site.mail_dir);
     mkdir(site.static_dir);
+    mkdir(site.plugins_dir);
 
     for (InitStep step : steps) {
       step.run();