Only map numeric HTTP status codes to request meta-data

[danielmitterdorfer]

On some occasions, the Python Elasticsearch client returns a non-numeric status code (like "N/A" or "TIMEOUT") in TransportError.status_code. Rally uses this value to populate the request meta-data field http-status which is mapped as numeric.

To avoid mapping errors when writing data to the metrics store, we should explicitly check whether the status code returned by the Python client is numeric.