### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

Currently the VTL template for `request_templates` is not processed properly:

```
#set( $body = $input.json("$") )

#macro( loop $map )
{
    #foreach($key in $map.keySet())
        #set( $k = $util.escapeJavaScript($key) )
        #set( $v = $util.escapeJavaScript($map.get($key)).replaceAll("\\'", "'") )
        "$k": "$v"
        #if( $foreach.hasNext ) , #end
    #end
}
#end

{
    "enhancedAuthContext": #loop($context.authorizer),

    "headers": #loop($input.params().header),

    "query": #loop($input.params().querystring),

    "path": #loop($input.params().path),

    "identity": #loop($context.identity),

    "stageVariables": #loop($stageVariables),
}
EOF
}
```

results in,

```
'

{
    "enhancedAuthContext": {
    }
,

    "headers": {
    }
,

    "query": {
    }
,

    "path": {
    }
,

    "identity": {
    }
,

    "stageVariables": {
    }
,
}
'
```

### Expected Behavior

The template should be rendered and have the JSON properties.

### How are you starting LocalStack?

Custom (please describe below)

### Steps To Reproduce

#### How are you starting localstack (e.g., `bin/localstack` command, arguments, or `docker-compose.yml`)

    docker run localstack/localstack

#### Client commands (e.g., AWS SDK code snippet, or sequence of "awslocal" commands)

    awslocal s3 mb s3://mybucket


### Environment

```markdown
- OS: MacOS
- LocalStack: 0.14
```


### Anything else?

This example can be used to validate.
https://github.com/localstack/localstack-terraform-samples/tree/master/apigateway-lambda-velocity
if uri.startswith("arn:aws:apigateway:") and ":dynamodb:action" in uri:
            
            table_name = uri.split(":dynamodb:action")[1].split("&Table=")[1]
            action = uri.split(":dynamodb:action")[1].split("&Table=")[0]
            if "PutItem" in action and method == "PUT":
                response_template = response_templates.get("application/json")
                if response_template is None:
                    msg = "Invalid response template defined in integration response."
                    LOG.info("%s Existing: %s", msg, response_templates)
                    return make_error_response(msg, 404)
                response_template = json.loads(response_template)
                if response_template["TableName"] != table_name:
                    msg = "Invalid table name specified in integration response template."
                    return make_error_response(msg, 404)
                dynamo_client = aws_stack.connect_to_resource("dynamodb")
                table = dynamo_client.Table(table_name)
                event_data = {}
                data_dict = json.loads(data)
                for key, _ in response_template["Item"].items():
                    event_data[key] = data_dict[key]
                table.put_item(Item=event_data)
                response = requests_response(event_data)
                return response
        else:
            raise Exception(
                'API Gateway action uri "%s", integration type %s not yet implemented'
                % (uri, integration_type)
            )
    elif integration_type in ["HTTP_PROXY", "HTTP"]:
        if ":servicediscovery:" in uri: