Do not programmatically set "artemis.instance.etc" system property

If the "artemis.instance.etc" system property is set programmatically it will break examples which create multiple brokers because when the "stop" command is run it won't find the right "etc" dir for the stop file which means brokers will potentially be left running. This isn't a problem when such an example is run by itself, but it is a problem when multiple examples are run at once (e.g. using mvn -Pexamples verify from the examples directory).