**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
self.set_up_operator()  
    def test_init(self):
        self.assertEqual(self.ecs.region_name, 'eu-west-1')
        self.assertEqual(self.ecs.task_definition, 't')
        self.assertEqual(self.ecs.aws_conn_id, None)
        self.assertEqual(self.ecs.cluster, 'c')
        self.assertEqual(self.ecs.overrides, {})
        self.ecs.get_hook()
        self.assertEqual(self.ecs.hook, self.aws_hook_mock.return_value)
        self.aws_hook_mock.assert_called_once()
    def test_template_fields_overrides(self):
        self.assertEqual(self.ecs.template_fields, ('overrides',))
    @parameterized.expand(
        [
            ['EC2', None],
            ['FARGATE', None],
            ['EC2', {'testTagKey': 'testTagValue'}],
            ['', {'testTagKey': 'testTagValue'}],
        ]
    )
    @mock.patch.object(ECSOperator, '_wait_for_task_ended')
    @mock.patch.object(ECSOperator, '_check_success_task')
    def test_execute_without_failures(self, launch_type, tags, check_mock, wait_mock):