<!-- 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`


self.assertEqual(self.RESOURCENOTFOUND_MESSAGE % lambda_api.func_arn(self.FUNCTION_NAME),
                             result['message'])
    def test_create_alias_returns_error_if_already_exists(self):
        self._create_function(self.FUNCTION_NAME)
        self.client.post('{0}/functions/{1}/versions'.format(lambda_api.PATH_ROOT, self.FUNCTION_NAME))
        data = json.dumps({'Name': self.ALIAS_NAME, 'FunctionVersion': '1', 'Description': ''})
        self.client.post('{0}/functions/{1}/aliases'.format(lambda_api.PATH_ROOT, self.FUNCTION_NAME), data=data)
        response = self.client.post('{0}/functions/{1}/aliases'.format(lambda_api.PATH_ROOT, self.FUNCTION_NAME),
                                    data=data)
        result = json.loads(response.get_data())
        alias_arn = lambda_api.func_arn(self.FUNCTION_NAME) + ':' + self.ALIAS_NAME
        self.assertEqual(self.ALIASEXISTS_EXCEPTION, result['__type'])
        self.assertEqual(self.ALIASEXISTS_MESSAGE % alias_arn,
                         result['message'])
    def test_update_alias(self):
        self._create_function(self.FUNCTION_NAME)
        self.client.post('{0}/functions/{1}/versions'.format(lambda_api.PATH_ROOT, self.FUNCTION_NAME))
        self.client.post('{0}/functions/{1}/aliases'.format(lambda_api.PATH_ROOT, self.FUNCTION_NAME),
                         data=json.dumps({
                             'Name': self.ALIAS_NAME, 'FunctionVersion': '1', 'Description': ''}))
        response = self.client.put('{0}/functions/{1}/aliases/{2}'.format(lambda_api.PATH_ROOT, self.FUNCTION_NAME,
                                                                          self.ALIAS_NAME),
                                   data=json.dumps({'FunctionVersion': '$LATEST', 'Description': 'Test-Description'}))
        result = json.loads(response.get_data())
        result.pop('RevisionId', None)  
        expected_result = {'AliasArn': lambda_api.func_arn(self.FUNCTION_NAME) + ':' + self.ALIAS_NAME,
                           'FunctionVersion': '$LATEST', 'Description': 'Test-Description',
                           'Name': self.ALIAS_NAME}
        self.assertDictEqual(expected_result, result)
    def test_update_alias_on_non_existant_function_returns_error(self):