Allow choice of Keyring to be configured in YAML

This uses a Dagger-provided map of Keyring implementations, with two currently available,
"KMS" and "Dummy". The active keyring is configured in the YAML file, so we no longer
require MOE directives to choose which one to use for internal/external builds.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=216898058

java/google/registry/keyring/KeyringModule.java

@@ -0,0 +1,41 @@
+// Copyright 2018 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.keyring;
+
+
+import static com.google.common.base.Preconditions.checkState;
+import dagger.Module;
+import dagger.Provides;
+import google.registry.config.RegistryConfig.Config;
+import google.registry.keyring.api.Keyring;
+import java.util.Map;
+import javax.inject.Singleton;
+
+/** Dagger module for {@link Keyring} */
+@Module
+public final class KeyringModule {
+
+  @Provides
+  @Singleton
+  public static Keyring provideKeyring(
+      Map<String, Keyring> keyrings, @Config("activeKeyring") String activeKeyring) {
+    checkState(
+        keyrings.containsKey(activeKeyring),
+        "Invalid Keyring %s is configured; valid choices are %s",
+        activeKeyring,
+        keyrings.keySet());
+    return keyrings.get(activeKeyring);
+  }
+}