ant does not finish because it started task service as a java process and it waits till it finishes. I don't think you need to start standalone task server as you use one that is deployed to JBoss. You could remove start.human.task target from depends attribute of start.demo.db task and then ant target should complete after jboss was started.
As it comes to the other part of the thread it seems little bit of misconfiguration of persistence, I believe that you persistence.xml for task uses data source that is defined in standalone.xml which uses jbpm5 user. So if that's the case either create new data source for task db or use same configuration as mentioned in docs - define connection details as properties in persistence.xml and ensure that <nonjta-data-source> or <jta-data-source> is removed from task service persistence.xml
HTH