InfluxDB metric source - check that queries contain just 1 column

[InfluxDB metric source](https://github.com/google/ts-bridge/blob/master/influxdb/README.md#metric-source-influxdb) is configured by specifying a query, this query must have 1 column, so that it returns just 1 value.
We're not checking this in [validateQuery](https://github.com/google/ts-bridge/blob/master/influxdb/config.go) , as a result we throw errors that are not very intuitive (like "cannot create metric '': not an App Engine context "