<!-- Love localstack? Please consider supporting our collective:
👉  https://opencollective.com/localstack/donate -->

# Type of request: This is a bug report

# Detailed description
When running a java lambda in localstack with `LAMBDA_EXECUTOR=docker` and  `LAMBDA_JAVA_OPTS="-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=5005"` (replaced `_debug_port_` with `5005`), the port is not opened in the lambda execution docker container.

Looking at `Util.get_java_opts` in `localstack/services/awslambda/lambda_executors.py`, `debug_java_port` is not set in case `_debug_port_` is set

```
class Util:
    debug_java_port = False

    @classmethod
    def get_java_opts(cls):
        opts = config.LAMBDA_JAVA_OPTS or ''
        if '_debug_port_' in opts:
            if not cls.debug_java_port:
                cls.debug_java_port = get_free_tcp_port() ### <<< debug_java_port is only set here
            opts = opts.replace('_debug_port_', ('%s' % cls.debug_java_port))
        ### In other case, `cls.debug_java_port` is not set.
        return opts
```
## Expected behavior

`debug_java_port` should be set to the configured port

## Actual behavior

`debug_java_port` is empty

# Steps to reproduce
docker-compose file
```
version: '2.1'
services:
  localstack:
    container_name: localstack
    image: localstack/localstack
    privileged: true
    ports:
      - "4567-4582:4567-4582"
      - "9070:9070"
    environment:
      SERVICES: sqs,lambda
      DOCKER_HOST: unix:///var/run/docker.sock
      PORT_WEB_UI: 9070
      LAMBDA_EXECUTOR: docker
      LAMBDA_JAVA_OPTS: "-agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=5005"
      DEBUG: 1
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock
      - /tmp/localstack:/tmp/localstack
```

## Command used to start LocalStack

`docker-compose up`


result = lambda_api.get_event_sources(source_arn='the_arn')
            self.assertEqual(len(result), 1)
            self.assertEqual(result[0].get('UUID'), self.TEST_UUID)
            
            result = lambda_api.get_event_sources(source_arn='the_')
            self.assertEqual(len(result), 0)
    def test_get_event_sources_with_paths(self):
        with self.app.test_request_context():
            lambda_api.event_source_mappings.append(
                {
                    'UUID': self.TEST_UUID,
                    'EventSourceArn': 'the_arn/path/subpath'
                })
            
            result = lambda_api.get_event_sources(source_arn='the_arn')
            self.assertEqual(len(result), 1)
            result = lambda_api.get_event_sources(source_arn='the_arn/path')
            self.assertEqual(len(result), 1)
    def test_delete_event_source_mapping(self):
        with self.app.test_request_context():
            lambda_api.event_source_mappings.append({'UUID': self.TEST_UUID})
            result = lambda_api.delete_event_source_mapping(self.TEST_UUID)
            self.assertEqual(json.loads(result.get_data()).get('UUID'), self.TEST_UUID)
            self.assertEqual(0, len(lambda_api.event_source_mappings))
    def test_invoke_RETURNS_415_WHEN_not_json_input(self):
        with self.app.test_request_context() as context:
            context.request._cached_data = '~notjsonrequest~'
            response = lambda_api.invoke_function(self.FUNCTION_NAME)
            self.assertEqual('415 UNSUPPORTED MEDIA TYPE', response.status)
    def _request_response(self, context):