Set KmsKeyring as the default Keyring

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=161113761

java/google/registry/model/registry/Registry.java

@@ -385,6 +385,7 @@ public class Registry extends ImmutableObject implements Buildable {
   TimedTransitionProperty<Money, BillingCostTransition> eapFeeSchedule =
       TimedTransitionProperty.forMapify(DEFAULT_EAP_BILLING_COST, BillingCostTransition.class);
 
+  /** Marksdb LORDN service username (password is stored in Keyring)  */
   String lordnUsername;
 
   /** The end of the claims period (at or after this time, claims no longer applies). */

java/google/registry/model/server/ServerSecret.java

@@ -35,6 +35,7 @@ import java.util.concurrent.ExecutionException;
 @Entity
 @Unindex
 @NotBackedUp(reason = Reason.AUTO_GENERATED)
+// TODO(b/27427316): Replace this with an entry in KMSKeyring
 public class ServerSecret extends CrossTldSingleton {
 
   /**