Protect reload plugin command with allow remote admin option

fd03350b9e introduced a new option
to disable remote plugin administration.

Require reloading plugins to be explicitly allowed with:

  plugins.allowRemoteAdmin = true

Change-Id: I2946202dd40af8b4197212fb15a488b166883d8a

gerrit-sshd/src/main/java/com/google/gerrit/sshd/commands/PluginReloadCommand.java

@@ -41,6 +41,9 @@ final class PluginReloadCommand extends SshCommand {
 
   @Override
   protected void run() throws UnloggedFailure {
+    if (!loader.isRemoteAdminEnabled()) {
+      throw die("remote plugin administration is disabled");
+    }
     if (names == null || names.isEmpty()) {
       loader.rescan();
     } else {