influxdb output: If all write fails, trigger a reconnect

closes #836

plugins/outputs/influxdb/influxdb.go

@@ -191,6 +191,12 @@ func (i *InfluxDB) Write(metrics []telegraf.Metric) error {
 			break
 		}
 	}
+
+	// If all of the writes failed, create a new connection array so that
+	// i.Connect() will be called on the next gather.
+	if err != nil {
+		i.conns = make([]client.Client)
+	}
 	return err
 }