[BROOKLYN-92] Configure brokerName for ActiveMQ

Issue raised in jira: https://issues.apache.org/jira/browse/BROOKLYN-92
Will `parent` ever be null? I think it's an error if an `ActiveMQDestination` has no `ActiveMQBroker` parent. So perhaps this whole method could be replaced with a simple `return getParent().getAttribute(ActiveMQBroker.BROKER_NAME)`.