systemPropertyVariables configuration option

This patch adds a `systemPropertyVariables` configuration option, which allows you to define system properties to be set when running a module.

This is particularly useful for setting the `org.vertx.logger-delegate-factory-class-name` property (my use case).

The option name and concept comes from the Apache Maven Surefire plugin (https://maven.apache.org/surefire/maven-surefire-plugin/examples/system-properties.html).