Check that a certain number of instances in a role are running.

def check_running(self, role, number):
    """
    Check that a certain number of instances in a role are running.
    """
    instances = self.get_instances_in_role(role, "running")
    if len(instances) != number:
      print "Expected %s instances in role %s, but was %s %s" % \
        (number, role, len(instances), instances)
      return False
    else:
      return instances