This PR is replacing the metrics.influxdb.host.tag cmd-line flag with metrics.influxdb.tags - a comma-separated key/value tags, that are passed to the InfluxDB reporter, so that we can index measurements with multiple tags, and not just one host tag.

This will be useful for Swarm, where we want to index measurements not just with the host tag, but also with bzzkey and git commit version (for long-running deployments).