**Description**

In similar fashion to https://github.com/apache/airflow/pull/4083, it would be helpful for Airflow to reattach itself to the ECS Task rather than letting another instance to start. However, instead of making this the default behavior, it would be better to use a `reattach` flag.

**Use case / motivation**

Allow Airflow the option to reattach to an existing ECS task when a restart happens, which would avoid having "rogue" tasks.
tests/providers/amazon/aws/operators/test_ecs.py
check_mock.assert_called_once_with()
        self.assertEqual(
            self.ecs.arn, 'arn:aws:ecs:us-east-1:012345678910:task/d8c67b3c-ac87-4ffe-a847-4785bc3a8b55'
        )
    def test_execute_with_failures(self):
        client_mock = self.aws_hook_mock.return_value.get_conn.return_value
        resp_failures = deepcopy(RESPONSE_WITHOUT_FAILURES)
        resp_failures['failures'].append('dummy error')
        client_mock.run_task.return_value = resp_failures
        with self.assertRaises(AirflowException):
            self.ecs.execute(None)
        self.aws_hook_mock.return_value.get_conn.assert_called_once()
        client_mock.run_task.assert_called_once_with(
            cluster='c',
            launchType='EC2',
            overrides={},
            startedBy=mock.ANY,  
            taskDefinition='t',
            group='group',
            placementConstraints=[{'expression': 'attribute:ecs.instance-type =~ t2.*', 'type': 'memberOf'}],
            placementStrategy=[{'field': 'memory', 'type': 'binpack'}],
            networkConfiguration={
                'awsvpcConfiguration': {
                    'securityGroups': ['sg-123abc'],
                    'subnets': ['subnet-123456ab'],
                }
            },
            propagateTags='TASK_DEFINITION',
        )
    def test_wait_end_tasks(self):