Bugfix for obfuscated secret file location on remote servers

Detect alternate location of secret file when it is moved into the `/config/rubber` dir on remote servers.

If you named your secret file as `/secret/rubber-secret.yml`, this would work by accident since it would be picked up by the `read_config method` (which reads all `rubber-xxx.yml` in the dif), not the `read_secret_config method`