remove hardcoded iam sign endpoint used only for test setup.

Author: zhumin8

oauth2_http/java/com/google/auth/oauth2/ComputeEngineCredentials.java

@@ -95,9 +95,6 @@ public class ComputeEngineCredentials extends GoogleCredentials
 
   static final String DEFAULT_METADATA_SERVER_URL = "http://metadata.google.internal";
 
-  static final String SIGN_BLOB_URL_FORMAT =
-      "https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/%s:signBlob";
-
   // Note: the explicit `timeout` and `tries` below is a workaround. The underlying
   // issue is that resolving an unknown host on some networks will take
   // 20-30 seconds; making this timeout short fixes the issue, but

oauth2_http/javatests/com/google/auth/oauth2/MockMetadataServerTransport.java

@@ -325,7 +325,8 @@ protected boolean isGetServiceAccountsUrl(String url) {
   protected boolean isSignRequestUrl(String url) {
     return serviceAccountEmail != null
         && url.equals(
-            String.format(ComputeEngineCredentials.SIGN_BLOB_URL_FORMAT, serviceAccountEmail));
+            String.format(
+                IamUtils.IAM_SIGN_BLOB_ENDPOINT_FORMAT, "googleapis.com", serviceAccountEmail));
   }
 
   protected boolean isIdentityDocumentUrl(String url) {