LOG_DIR not set causing FileNotFoundException: /kafka-rest.log (Permission denied)

To fix it, edit `bin/kafka-rest-run-class` and add:
```
if [ "x$LOG_DIR" = "x" ]; then
  LOG_DIR="$base_dir/logs"
fi
```
just before
```
if [[ -n $LOG_DIR ]]; then
    [[ -d $LOG_DIR ]] || mkdir -p "$LOG_DIR"
    KAFKAREST_LOG4J_OPTS="-Dkafka-rest.log.dir=$LOG_DIR ${KAFKAREST_LOG4J_OPTS}"
fi
```
