Describe the bug
The road-traffic-demo POM contains a property for configuring the port to use for Gaffer's REST API.

However, changing this property doesn't in fact change the port used.

To Reproduce
Steps to reproduce the behaviour:

1. Edit the port used in the POM or override the POM property manually.
2. Run the demo using mvn install -Proad-traffic-demo -pl :road-traffic-demo.
3. The REST API will start using port 8080, and not the standalone-port value.

Expected behaviour
Setting the standalone-port property value will set the REST API port used when the demo is run.