Move registerEmailPrivateKey to secure.config

This is the last column in the system_config table that is needed by
the stand-alone daemon installation format. Later we can look into
dropping the entire system_config table.

Change-Id: I8448c79e959b465e370a10a7fa6751c200c1b1a0

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

@@ -18,6 +18,7 @@ import static com.google.gerrit.pgm.init.InitUtil.dnOf;
 
 import com.google.gerrit.pgm.util.ConsoleUI;
 import com.google.gerrit.reviewdb.AuthType;
+import com.google.gwtjsonrpc.server.SignedToken;
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
 
@@ -80,5 +81,9 @@ class InitAuth implements InitStep {
         break;
       }
     }
+
+    if (auth.getSecure("registerEmailPrivateKey") == null) {
+      auth.setSecure("registerEmailPrivateKey", SignedToken.generateRandomKey());
+    }
   }
 }