Use @DefaultCredential for Cloud API access in GAE

This change completes the switch to @DefaultCredential for
all use cases in GAE.

Impacted modules:
- IcannReporting
- CreateCdnsTld command
- LoadSnapshot command.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=213511730

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

@@ -274,6 +274,11 @@ misc:
   # hosts from being used on domains.
   asyncDeleteDelaySeconds: 90
 
+cloudDns:
+  # CloudDns testing config. Set both properties to null in Production.
+  rootUrl: https://staging-www.sandbox.googleapis.com
+  servicePath: dns/v2beta1_staging/projects/
+
 beam:
   # The default zone to run Apache Beam (Cloud Dataflow) jobs in.
   defaultJobZone: us-east1-c

java/google/registry/config/files/nomulus-config-production-sample.yaml

@@ -57,5 +57,9 @@ registrarConsole:
 misc:
   sheetExportId: placeholder
 
+cloudDns:
+  rootUrl: null
+  servicePath: null
+
 kms:
   projectId: placeholder