test/rgw/multisite: test error handling of forwarded iam:DeleteRole

test/rgw/multisite: test error handling of forwarded iam:DeleteRole

DeleteRole's conflict handling happens after forwarding, so use
test_role_delete_sync() to test that forwarded 409 Conflict errors
preserve the DeleteConflict code and error message

without the fix to forward_iam_request_to_master(), DeleteRole instead
fails with:

> botocore.exceptions.ClientError: An error occurred (BucketNotEmpty) when calling the DeleteRole operation: None

Signed-off-by: Casey Bodley <cbodley@redhat.com>
(cherry picked from commit 3e9faa4de0d210623f7482aa243384ed1eaf1350)