Spark job server Gradle task should pass along system properties

The Spark job server Gradle task (:beam-runners-spark-job-server:runShadow) should pass on system properties, as Spark applications often rely on system properties for configuration. For example, if I want to enable logging, I currently have to set it programatically: 

System.setProperty("spark.eventLog.enabled", "true");

whereas I should be able to just do this:

./gradlew :beam-runners-spark-job-server:runShadow -Dspark.eventLog.enabled=true