It is my understanding that setting an environment varialbe `TEST_AWS_ACCOUNT_ID` for localstack sets that as the AWS account for the user in localstack.

However, setting it does not actually change any ARNs. For example:

1. Start localstack using `TEST_AWS_ACCOUNT_ID="111111111111" DEBUG=1 localstack start`
2. Deploy anything with an ARN (in this example I deployed a lambda)
3. Check the ARN. Example output for lambda:
```json
$ awslocal lambda list-functions
{
    "Functions": [
        {
            "FunctionName": "localtest-local-hello",
            "FunctionArn": "arn:aws:lambda:us-east-1:000000000000:function:localtest-local-hello",
            "Runtime": "nodejs10.x",
            "Role": "test_role",
            "Handler": "handler.hello",
            "CodeSize": 3077,
            "Description": "",
            "Timeout": 6,
            "MemorySize": 1024,
            "LastModified": "2019-11-06T06:43:53.451+0000",
            "CodeSha256": "V77gR13o90ET7+XquXBBc3Rxr1qDu1bE7S+A0Wbx5ys=",
            "Version": "$LATEST",
            "TracingConfig": {
                "Mode": "PassThrough"
            },
            "RevisionId": "3d4ed679-c1a9-4b7e-ad9e-f27e8543c5e9"
        }
    ]
}
```

ARN is still 000000000000
import re
import json
import logging
import requests
from flask import Response as FlaskResponse
from six.moves.urllib_parse import urljoin
from requests.models import Response
from localstack.constants import APPLICATION_JSON, PATH_USER_REQUEST
from localstack.config import TEST_KINESIS_URL, TEST_SQS_URL
from localstack.utils import common
from localstack.utils.aws import aws_stack
from localstack.utils.common import to_str, to_bytes
from localstack.utils.analytics import event_publisher
from localstack.services.kinesis import kinesis_listener
from localstack.services.awslambda import lambda_api
from localstack.services.apigateway import helpers
from localstack.services.generic_proxy import ProxyListener
from localstack.utils.aws.aws_responses import flask_to_requests_response, requests_response
from localstack.services.apigateway.helpers import (get_resource_for_path,
    handle_authorizers, extract_query_string_params,
    extract_path_params, make_error, get_cors_response)
LOGGER = logging.getLogger(__name__)
PATH_REGEX_AUTHORIZERS = r'^/restapis/([A-Za-z0-9_\-]+)/authorizers(\?.*)?'
PATH_REGEX_RESPONSES = r'^/restapis/([A-Za-z0-9_\-]+)/gatewayresponses(/[A-Za-z0-9_\-]+)?(\?.*)?'
PATH_REGEX_USER_REQUEST = r'^/restapis/([A-Za-z0-9_\-]+)/([A-Za-z0-9_\-]+)/%s/(.*)$' % PATH_USER_REQUEST
GATEWAY_RESPONSES = {}
class ProxyListenerApiGateway(ProxyListener):