title: Gerrit plugin-manager doesn't show plugins
body: <p>I have a <code>Gerrit v2.15</code> on a linux machine and I'm the administrator in Gerrit.</p>
  
  <p>I installed the Gerrit plugin-manager under gerrit/plugins with command :</p>
  
  <p><code>wget https://gerrit-ci.gerritforge.com/view/Plugins-stable-2.15/job/plugin-plugin-manager-bazel-master-stable-2.15/lastSuccessfulBuild/artifact/bazel-genfiles/plugins/plugin-manager/plugin-manager.jar</code></p>
  
  <p>But nothing uploads in Gerrit UI in tab Plugins. I only see the Installed, as before.</p>
  
  <p>In gerrit/logs/<strong>error.log</strong> I can read out the following:</p>
  
  <pre><code>[2018-06-15 08:54:26,366] [main] INFO  com.google.gerrit.server.plugins.PluginLoader : Loaded plugin plugin-manager, version v2.15-1-g27c3d2935d
  [2018-06-15 08:54:26,367] [plugin-manager-preloader] INFO  com.googlesource.gerrit.plugins.manager.OnStartStop : Start-up: pre-loading list of plugins from registry
  [2018-06-15 08:54:26,902] [main] INFO  com.google.gerrit.server.git.GarbageCollectionRunner : Ignoring missing gc schedule configuration
  [2018-06-15 08:54:26,902] [main] INFO  com.google.gerrit.server.config.ScheduleConfig : accountDeactivation schedule parameter "accountDeactivation.interval" is not configured
  [2018-06-15 08:54:26,902] [main] INFO  com.google.gerrit.server.change.ChangeCleanupRunner : Ignoring missing changeCleanup schedule configuration
  [2018-06-15 08:54:26,946] [main] INFO  com.google.gerrit.sshd.SshDaemon : Started Gerrit SSHD-CORE-1.6.0 on *:29418
  [2018-06-15 08:54:26,949] [main] INFO  org.eclipse.jetty.server.Server : jetty-9.3.18.v20170406
  [2018-06-15 08:54:28,040] [main] INFO  com.googlesource.gerrit.plugins.gitiles.HttpModule : No /opt/gerrit/etc/gitiles.config; assuming defaults
  [2018-06-15 08:54:28,847] [main] INFO  org.eclipse.jetty.server.handler.ContextHandler : Started o.e.j.s.ServletContextHandler@62672ad5{/,null,AVAILABLE}
  [2018-06-15 08:54:28,869] [main] INFO  org.eclipse.jetty.server.AbstractConnector : Started ServerConnector@38503309{HTTP/1.1,[http/1.1]}{127.0.0.1:8081}
  [2018-06-15 08:54:28,890] [main] INFO  org.eclipse.jetty.server.Server : Started @20170ms
  [2018-06-15 08:54:28,893] [main] INFO  com.google.gerrit.pgm.Daemon : Gerrit Code Review 2.15.1 ready
  [2018-06-15 08:54:44,819] [plugin-manager-preloader] INFO  com.googlesource.gerrit.plugins.manager.OnStartStop : 45 plugins successfully pre-loaded
  </code></pre>
  
  <p>What have i missed?</p>
  
  <p>Is it some permission settings that is wrong or configuration in gerrit.config that has to be done?</p>
  
  <p>Directory gerrit/plugin has permissions set to: drwxr-xr-x</p>
  
  <p><em>Note</em> : I've tried restarting Gerrit.</p>
  
  <p><em>Good to know</em>: We are using nginx. That might be useful info. 
  I know very little about how nginx works.</p>
  
  <p><a href="https://i.stack.imgur.com/QaLoS.png" rel="nofollow noreferrer">What it looks like in the UI</a></p>