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We have several Kinesis streams set up with event notifications that trigger lambdas.  When the LAMBDA_EXECUTOR is set to 'docker' and we send a large number or records to Kinesis, multiple Lambdas will execute, but currently they are failing to create with error:

```
Exception: Lambda process returned error status code: 1. Output:
time="2019-09-18T17:28:11Z" level=error msg="error getting events from daemon: net/http: request canceled" 
Error response from daemon: driver failed programming external connectivity on endpoint busy_sanderson (a78e45a41d0ce5b0d8bab6ff54344fc34fe2b4969f55bf568e753121abc8759e): Bind for 0.0.0.0:38763 failed: port is already allocated
```

I believe this is due to the new debug_java_port introduced with this PR https://github.com/localstack/localstack/pull/1523

With this change, you are setting the port in the constructor by using a method known to have a race condition (https://github.com/localstack/localstack/blob/master/localstack/utils/common.py#L354)
(https://gist.github.com/gabrielfalcao/20e567e188f588b65ba2)

This should be changed to ensure unique ports are being issued across all lambdas


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):
        with self.app.test_request_context():
            result = json.loads(lambda_api.update_alias(self.FUNCTION_NAME, self.ALIAS_NAME).get_data())
            self.assertEqual(self.RESOURCENOTFOUND_EXCEPTION, result['__type'])
            self.assertEqual(self.RESOURCENOTFOUND_MESSAGE % lambda_api.func_arn(self.FUNCTION_NAME),