Set default of 'yes' for update_cache, as is in the documentation

packaging/apt_repository

@@ -333,7 +333,7 @@ def main():
         argument_spec=dict(
             repo=dict(required=True),
             state=dict(choices=['present', 'absent'], default='present'),
-            update_cache = dict(aliases=['update-cache'], type='bool'),
+            update_cache = dict(aliases=['update-cache'], type='bool', default='yes'),
         ),
         supports_check_mode=True,
     )