Use a constant for module source url in test

Author: ericzbeard

tests/unit/customizations/cloudformation/modules/url-template.yaml

@@ -1,6 +1,8 @@
+Constants:
+  ModuleSource: https://raw.githubusercontent.com/aws/aws-cli/2f0143bab567386b930322b2b0e845740f7adfd0/tests/unit/customizations/cloudformation/modules
 Modules:
   Content:
-    Source: https://raw.githubusercontent.com/aws/aws-cli/2f0143bab567386b930322b2b0e845740f7adfd0/tests/unit/customizations/cloudformation/modules/basic-module.yaml 
+    Source: !Sub ${Constant::ModuleSource}/basic-module.yaml  
     Properties:
       Name: foo
     Overrides: