Check task state when calculating API URL

[Michael Coles]

When attempting to find the scheduler's API port, we use:

        client = self.ctx.marathon_client()
        tasks = client.get_tasks(self.ctx.framework)
        if len(tasks) != 0:

and simply assume that the first task in the list is the scheduler. However, it looks like marathon now also includes any lost tasks it knows about.

We should inspect each task object's state field for "TASK_RUNNING" before attempting to test the healthcheck.