Add config parameters to point at us to use cloud-dns staging

Add cloudDns.{rootUrl, servicePath} to allow us to point an environment at the
Cloud DNS staging API for testing.  Make sandbox and alpha point to staging.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=170340859

java/google/registry/config/RegistryConfig.java

@@ -331,6 +331,18 @@ public final class RegistryConfig {
       return config.datastore.eppResourceIndexBucketsNum;
     }
 
+    @Provides
+    @Config("cloudDnsRootUrl")
+    public static Optional<String> getCloudDnsRootUrl(RegistryConfigSettings config) {
+      return Optional.fromNullable(config.cloudDns.rootUrl);
+    }
+
+    @Provides
+    @Config("cloudDnsServicePath")
+    public static Optional<String> getCloudDnsServicePath(RegistryConfigSettings config) {
+      return Optional.fromNullable(config.cloudDns.servicePath);
+    }
+
     /**
      * Returns size of Google Cloud Storage client connection buffer in bytes.
      *

java/google/registry/config/RegistryConfigSettings.java

@@ -25,6 +25,7 @@ public class RegistryConfigSettings {
   public OAuth oAuth;
   public RegistryPolicy registryPolicy;
   public Datastore datastore;
+  public CloudDns cloudDns;
   public Caching caching;
   public IcannReporting icannReporting;
   public Rde rde;
@@ -96,6 +97,12 @@ public class RegistryConfigSettings {
     public String projectId;
   }
 
+  /** Configuration for Cloud DNS. */
+  public static class CloudDns {
+    public String rootUrl;
+    public String servicePath;
+  }
+
   /** Configuration for caching. */
   public static class Caching {
     public int singletonCacheRefreshSeconds;

java/google/registry/config/files/default-config.yaml

@@ -110,6 +110,15 @@ datastore:
   # doubles after each failure).
   baseOfyRetryMillis: 100
 
+cloudDns:
+  # The root url for the Cloud DNS API.  Set this to a non-null value to
+  # override the default API server used by the googleapis library.
+  rootUrl: null
+
+  # The service endpoint path for the Cloud DNS API.  Set this to a non-null
+  # value to override the default API path used by the googleapis library.
+  servicePath: null
+
 caching:
   # Length of time that a singleton should be cached before expiring.
   singletonCacheRefreshSeconds: 600