Configure signed push globally in Gerrit config

Instead of depending on the standard C git option
receive.certNonceSeed to enable signed push, require a separate option
receive.enableSignedPush. This avoids tying enabling the functionality
to a particular implementation of nonce validation, and in the common
case we can just use a new random seed on startup.

The standard C git keys receive.certNonceSeed and
receive.certNonceSlop are supported, but read them from the global
Gerrit config (preferably secure.config).

Change-Id: Ib38d16052e8896e860e4d9639c62b2023784ef97

gerrit-server/src/test/java/com/google/gerrit/testutil/InMemoryModule.java

@@ -94,6 +94,8 @@ public class InMemoryModule extends FactoryModule {
     cfg.setInt("index", "lucene", "testVersion",
         ChangeSchemas.getLatest().getVersion());
     cfg.setInt("sendemail", null, "threadPoolSize", 0);
+    cfg.setBoolean("receive", null, "enableSignedPush", false);
+    cfg.setString("receive", null, "certNonceSeed", "sekret");
   }
 
   private final Config cfg;