The `Gitlab::GitalyClient::StorageSettings#gitaly_address` currently
fails, because the GitLab configuration in production defines the
gitaly address entry for each node using a string key, and the method
actually tries to fetch the value based on a symbolic key.

Interestingly enough, the `config/gitlab.yml.example` uses a symbolic
key.

Use `with_indifferent_access` instead of fallback.

Add spec tests and fix fall-back settings fetch.